Jeremy Corbyn has addressed supporters from the top of a fire engine after a planned rally spilled out into the street because so many people turned up.
At a noisy and excitable rally, the Labour leadership contender made a series of speeches to the packed main hall, a council chamber and outside Camden Town Hall.
The event, which was also addressed by Ken Livingstone, Owen Jones and Mark Serwotka, saw long queues of people, some of whom had booked tickets while others arrived hoping to get in on the night.
Mr Corbyn has been installed by some as an improbable front runner in the contest despite him getting on to the ballot paper at the last minute after securing nominations from MPs who had no intention of backing him in the main contest.
The leadership battle is due to conclude on September 12.
Labour's leadership candidates, who also include Andy Burnham, Yvette Cooper and Liz Kendall, had been due to take a week off campaigning before ballot papers are sent to Labour Party members and supporters.
Arriving to a lengthy standing ovation, Mr Corbyn said Labour had been wrong to accept Conservative arguments on the economy and blamed it for May's devastating general election defeat.
He said: "It's an incredible evening when we have had to have overflow meetings in the council chamber, downstairs and on top of the FBU fire engine outside."
Mr Corbyn told activists: "After September 12, whatever the result, don't go away and say that was a good summer 2015 ... stay together to defend what we have got but above all, stay together to bring about a country that doesn't thrive on inequality and injustice and poverty.
"A country that instead genuinely does care for all, that does provide public services that are good for all, does expand our economy, does take back into public ownership crucial monopolies like railways, Royal Mail and so many others, that doesn't give up the fruits of what we are trying to do and doesn't allow this Tory Government to take away our workers' rights and our trade union rights.
"Come together in that sense of strength and I am sure some people who have made slightly negative comments about what we are saying at the moment within the confines of Parliament will understand this is an open and democratic process that has brought lots of people together on the basis of hope.
"That voice cannot be silenced. That voice cannot be stopped. That power cannot be denied - together we are very strong. The alternative of staying home and shouting at the television will achieve absolutely nothing."
In a 30-minute speech made without notes, Mr Corbyn also joked it was the Tories who looked like a throwback to the 1980s as he rejected claims he would take Labour back to the days of successive defeats to Conservative majority government.
And he attacked "disgusting language" used about the migrant crisis in Calais.
Mr Corbyn also told Channel 4 News: "A number of people are taking notice of our campaign where we are challenging the agenda of austerity, austerity which has led to greater inequality in our society, greater poverty in our society and now the budget and welfare reform bill which will deepen that.
"Surely we can do better than that, surely the Labour Party can do better than that, and offer a real economic alternative which is about expanding manufacturing industry, expanding our economy and a welfare state that eliminates poverty and the gross inequality that exists in Britain today."
Mr Corbyn refused to name anyone who would serve in his shadow cabinet, as his rise to be favourite provoked deep divisions within the party.
Shadow chancellor Chris Leslie claimed the veteran left-winger's strategy would push up inflation and interest rates, hitting the poorest hardest.
Mr Leslie, who said he would not serve in a shadow cabinet under Mr Corbyn, urged party members to "think carefully" about the impact of the Islington North MP's policies and whether he would be a credible candidate for Downing Street.
Allies of Mr Corbyn rallied to defend "Corbynomics" - the left-winger's economic plans.
Richard Murphy, an economic adviser to Mr Corbyn's campaign and of Tax Research UK, told BBC Radio 4's World At One that Mr Leslie has "got this completely wrong" as he defended the plan for "people's quantitative easing".