Campaigners claim "two to three thousand" gathered outside Parliament earlier in an effort to persuade MPs to back a referendum on the Lisbon Treaty.
Campaigners queued to lobby their MPs for a referendum
The I Want a Referendum group's supporters formed a long queue towards the Westminster Palace security booth.
The Tories and some Labour and Lib Dem MPs want a referendum on ratifying the treaty, which they say is largely the same as the discarded EU Constitution.
But the government says Parliament, not the public, should have the final say.
I Want a Referendum hopes to persuade more politicians to back its campaign, ahead of a vote next week on a Conservative amendment to the EU Treaty Bill, calling for a referendum.
Protesters outside Parliament held banners bearing the slogans "Politicians: where's our referendum?", "Let the people decide" and "Politicians: keep your referendum promise".
David Tucker, from Baslow, Derbyshire, said: "I'm here to defend the principle of a referendum. We should have our say."
Dorothy Collins, from Islington, north London, said: "I think it's essential that we are given the same chance to tell politicians what we feel as people in other countries."
Neil O'Brien, director of I Want a Referendum, said: "It's been very good. It's a better turnout than we had expected. People are becoming more and more interested in the issue."
The three main parties promised a public vote on the EU Constitution in the 2005 general election.
But the constitution was discarded later that year after the Dutch and French electorates rejected it. The new treaty was drawn up instead.
During prime minister's questions, Conservative leader David Cameron urged Gordon Brown to allow a free vote among Labour MPs.
He said: "Today there are hundreds of people marching on Parliament and asking for the referendum they were promised on the EU Constitution.
"Will he at least accept that it can't be right to ask his own members of Parliament - many of whom really feel a conscientious belief that they signed up to a manifesto - to vote against their consciences. Can that be right?"
He added: "If the prime minister is so confident of his position ... he should give them a free vote."
Mr Brown replied that the "constitutional concept" had been abandoned in Brussels adding: "This is an amending treaty and not a constitutional treaty."
He pointed out there were disagreements within the Conservative Party about whether it required a referendum and challenged Mr Cameron to say if he would continue to campaign for a referendum, even if the treaty was ratified across Europe.
The Liberal Democrat leadership says it will not back the Conservative amendment and has instead called for a referendum on the wider question of Britain's membership of the EU.
But on Tuesday, that topic was not called for debate, prompting an angry protest by Lib Dem foreign affairs spokesman Edward Davey, who was eventually ordered out of the Commons, followed by many Lib Dem MPs.
At prime minister's questions, Lib Dem leader Nick Clegg criticised "clapped out, 19th Century practices" which had prevented his party having that debate.
Mr Brown replied that there was no support for a referendum on EU membership, adding: "Welcome back. I hope this time you can stay long enough to hear the answers."
The EU Reform Treaty, signed by Prime Minister Gordon Brown and other national leaders in Lisbon in December, was designed to replace the constitution.
Ministers argue that no referendum is needed as the treaty merely amends the EU's existing constitution, rather than overwriting it - as the failed constitution would have done.
But the Tories, UKIP, SNP, Plaid Cymru, DUP and various groups, including I want a Referendum, say the two documents are substantially the same and that the public must have a say.