Nick Clegg addressing protesters: "The fact that you are out on the streets... is wonderful"
About a thousand protesters demanding electoral reform have gathered outside the building where senior Liberal Democrats were meeting in Westminster.
The demonstrators were from groups seeking proportional representation.
The Lib Dems were discussing a possible collaboration with the Tories who failed to secure a majority at the general election.
The protesters urged the party to stand firm in their long standing policy call for changes to the system.
PR was one of the Lib Dems' main demands for cooperation in government.
Lib Dem leader Nick Clegg was meeting his MPs and peers to discuss a power-sharing offer from the Conservatives.
He broke off from the talks to receive a petition from the demonstrators.
Mr Clegg told them: "It's in the interests of everybody in Britain for us to use this opportunity to usher in a new politics after the discredited politics of the past...
"Take it from me, reforming politics is one of the reasons I went into politics."
Pam Giddy, from the group Power 2010, said the campaigners were from a range of organisations seeking a "purple revolution".
Among them, campaigner and musician Billy Bragg said the hung Parliament was an opportunity to bring about "genuine electoral reform".
He said: "We don't like these negotiations going on behind closed doors. The votes that people cast on Thursday should have given us a coalition government.
"We need a consensus government to deal with the difficult choices we have ahead. That's what the British people voted for."
The Tories won most votes in the UK election but were short of a majority - Gordon Brown remains PM while Tories see if they can form a government.
Mr Clegg also spoke to the PM by phone on Friday. He has offered talks if Lib Dems cannot agree with the Tories.