Animal rights protesters were banned from holding a demonstration in Oxford city centre after a late decision by police.
Hundreds of people had been planning to meet at Carfax at lunchtime on Saturday to continue their campaign against the building of Oxford University's £18m biomedical research lab.
But they were greeted by a large number of police who told them to move on or be arrested after Deputy Chief Constable Alex Marshall invoked the ban under section 14 of the Public Order Act.
It meant they were restricted to protesting in Keble Road.
Mr Marshall made the order late on Thursday, saying protesters had not discussed their plans with police beforehand. Police claimed they told protesters before Saturday but members of the group Speak said they were unaware of it.
Three women were arrested on suspicion of breaching the order in the city centre. A 30-year-old man was also arrested on suspicion of obstructing the highway.
About 200 demonstrators demonstrated in Keble Road.
Thames Valley police spokesman Tim Prince said: "The decision to implement Section 14 of the Public Order Act was taken late on Thursday.
"The protesters were told of this. They did not want to enter into negotiations or discussions with us."
Mr Prince said he could not say on what basis Deputy Chief Constable Mr Marshall had made his decision.
Mel Broughton, of Speak, said: "We had no idea we would not be allowed to demonstrate in the city centre. I was threatened with arrest for holding a placard. I thought we lived in a democratic country.
"Now it seems we are not allowed to stand in Oxford city centre. It is a sad day for democracy.
"You've got to start asking yourself some serious questions about democracy after this."
He said Speak decided not to negotiate with the police over their plans because they were unhappy with how they had previously been treated by the force.
He added: "What we have seen the police do is completely and utterly over-react yet again.
"There were hundreds and hundreds of police officers spread across Oxford city centre trying to stop anyone from assembling anywhere.
"Quite clearly they had an agenda and that was to stop anyone speaking out against Oxford University and what's happening to animals."
Protesters from as far afield as London and Birmingham had travelled to Oxford to be part of the event.
Laurence Parker, from London, was given several warnings by police for walking with a placard in St Giles.
He said: "I came here to protest but I can't protest except in a designated spot where none of the public are.
Mr Parker added: "This was supposed to be about raising public awareness, but we can't do that if we're penned in in Keble Road."