A crowd of hundreds is gathered outside London's Southwark Cathedral, chanting, banging the doors, screaming to be let in to hear her speak. Inside the hall, the cacophony echoes around the beautifully-carved statues and the glorious stained glass, but the eyes of the congregation are all focused on one tiny figure. Sandwiched between a grinning Ken Livingstone and a solemn Gerry Adams is a diminutive woman with an almighty surname and the same smiling brown eyes as a man whose image is purported to be more recognisable than Jesus.

Like her father, Aleida Guevara is the living embodiment of resistance. As she scurries across the platform to a standing ovation, one can't help wondering how she can live up to this burden of expectation.

When Aleida speaks, she is indomitable. ''I am here to tell you,'' she announces, ''that socialism is still a possibility. There is an alternative way to achieve the world we hope for. Cuba represents that reality and Cuba is a beacon for the people of the world.''

For the 30,000 delegates at the European Social Forum, Cuba is a rock of inspiration, and the totemic image of Ernesto ''Che'' Guevara is as powerful here in London as it is in Havana. His followers have come to discuss human rights, the environment, social justice, globalisation and third-world debt. They are here, as Aleida wrote in her introduction to the Motorcycle Diaries: ''To put into practice Che's desire to create a world that is far more just.''

Aleida is here to encourage the world to call for the US to end the military and economic blockade which is strangling her island, and to let Cuba determine its own future. She is being hosted by the UK-based Cuban Solidarity Campaign, a support group, operating from the decaying offices of a battered brick building in London's Finsbury Park.

When I meet her there, it's a cold miserable day and the roof is leaking on to an old school desk beside us. The room is bare but for a draped Cuban flag. It seems a fitting venue to discuss the plight of a nation which has proudly struggled to survive with almost nothing.

''I am not an economist, so I will give you a clear example,'' she says, gripping her throat. ''A child of five, she had a diagnosis that she was bleeding from the digestive system. There is a specific medicine for that pathology, but the US has the exclusive rights on it. We have the money to buy the medicine, but no company would sell it to us because the US could then sanction that company. That is the blockade that we live.''

Aleida's last memory of her father, who died in the Bolivian jungle in 1967 after he joined a failed uprising against the government - was when she was six years old. At the time, she didn't know it was him because he was in disguise. He cradled her in his arms after she fell over and knocked her head. She recalls feeling a strong sense ''that this man loves me''.

Aleida, or Aleidita, was the first daughter of his second wife, Aleida March, born while Che was in Beijing. Che's ideals were taught to her by her mother and Fidel Castro, the man she calls ''uncle''.

Like her father, she became a paediatrician, specialising in allergies. At the age when he had headed through Latin America on his motorcycle, Aleida was completing her medical studies in the hospitals of Nicaragua.

At 42, she is now three years older than Che when he died and has two children, who she supports with her modest doctor's salary. The Cubans she calls ''the children of Che'' are dying in her arms every day. ''There are no words to describe this,'' she sighs. ''Because I'm also a mother, it's difficult for me to think that there's a medicine I could give to a child but I can't. It makes you indignant. But it gives you quite a strong force from inside. You become determined that nobody will dominate your life.''

It is this siege mentality, the spirit of solidarity and independence, which has inspired Aleida to forge her own identity as the woman we see today: an ambassador, a fierce campaigner and a ''militant of the Cuban communist party''. It was a duty she took on ''when I realised my surname could be used to make people listen to the realities of my people''.

She has travelled the world to raise awareness of ''the Cuban Five'', detained in a Florida jail on disputed espionage charges and has joined Castro in demanding that Guantanamo Bay be returned as Cuban territory. Most of all, she has fought to protect the exploitation of her father's famous image. ''We all feel very close to Che. A few months ago a little girl came up to me and said: 'You won't get upset if I tell you that your dad is also my dad?'. I said no problem, we'll just share him. That's common in Cuba. Children feel that he's part of them. We identify with him because he left clear objectives for any youth so that they could become a better human being.''

The Che Guevara institute in Havana has fought lengthy battles to keep his portrait off the side of vodka bottles and other ''capitalist enterprises''. But she is happy to see it ''on the chests of young men and women who respect him''.

Aleida is not enamoured with the most iconic image of them all - the face of Che in his beret, the guerrilla

commander. She prefers ''his more typical expressions''. She speaks of his charm, honesty and self-determination and is happy for him to symbolise the universal revolutionary. ''We fight against the myth, because the myth takes us further away from the real achievements of a real man. My father was not Jesus Christ.''

She encourages his followers to get to know the man through the diaries and manuscripts the Che Guevara Institute continues to publish, but when approached on more controversial topics about Che's life, she sticks to the established story.

In a new biography of Che, Glasgow University lecturer Mike Gonzalez criticises his politics, arguing that the attempted revolutions in Congo and Bolivia were a disaster because Che failed to rally the support of the peasants he was supposed to be liberating. ''That's not true,'' says Aleida.

Her defence of Che is deeply personal, although in the light of the resurgence of left-wing activity in Latin America, it is also an important political stance. Hugo Chavez's referendum victory in Venezuela was inspired by Cuba and has been heralded as the beginning of a new dawn of social change. Addressing a packed hall at the social forum, when the subject is raised, Aleida is triumphal: ''Cuba supports the revolution. We have more than 15,000 doctors in Venezuela, because it meets the basic objectives we look for in a Latin American country. That is that the people should be in charge of what they produce and should be able to use their natural resources for their own benefit.'' Cuba has supplied teachers, who, it is claimed, taught 1.3m Venezuelans to read and write. But Cuba's reputation as a benevolent force in Latin America has increased speculation that the US may be ready to take

military action on Cuba, to protect its interests in the free markets of other Latin American countries. Florida governor, Jeb Bush, said: ''After its success in Iraq, Washington should finish with the regime of Castro.''

When the subject is raised, Guevara accused the US of failing to respect different cultures. The crowd rewards her with a huge cheer, but afterwards she receives a more divided response. ''I'm happy that she is not just the daughter of Che Guevara,'' says Julian, a Frenchman.

But others are not so overwhelmed. ''I didn't like her. She was too demagogic,'' says Naila Vazquez, a student from Barcelona. ''She was very selective with her examples. She talks a lot about freedom [of Cubans] and democracy but she was side-stepping a lot of questions.''

The questions relate to Cuba's human rights record. Castro's

government is accused of stifling free speech, condoning abusive treatment of homosexuals and denying the public a right to free and fair elections.

Guevara gives a bullish defence. ''We had [a conventional] democracy and our problems were not resolved. Today we're beginning to realise our vision. We commit human errors, we're not perfect, but we have a right to determine our own future and solve our own problems.''

At times, she verges on propaganda, but, having arrived at the forum to rally support against the military might of the most powerful nation on earth, her aggression is somewhat understandable. Guevara is, after all, living on the frontline of the battle to save Cuban lives.

Her party politics have been

lauded and vilified, but her passionate dedication to her people is admirable. As she says of her papa: ''No man is perfect, but Jose Marti always said that honest men should never look for the stains on the sun. Because the sun gives us power.''

Visit the Cuban Solidarity Campaign at www.cuba-solidarity.org.uk