I'd heard impressive things said about London Citizens and their ability to mobilise vast numbers of people in pursuit of their collectivist agenda, but I don't think anything could have prepared me for their Autumn Assembly on Wednesday which was quite simply one of the most extraordinary pieces of political theater I've ever witnessed.

It was part Question Time, part civics lesson, part high-school talent show all delivered in the style of a pentecostal sermon - and it worked magnificently.

Two thousand community 'leaders', collectively representing 150 institutions with over 50,000 members, all piled into the vast multi-levelled Barbican theater. The turnout was impressive, but also the diversity. There were school groups, mosques, churches, students, university facilties and race-based organisations. Sat next to me were a group of elders from an East End mosque, a university society and a Chinese immigrants support-group.

The Assembly opened with a 'roll call' on stage where representatives of each of the 29 associated boroughs rattled off the list of groups from their area and called on them to 'stand up and be recognised' at which point between 5 and 50 people in the audience would spring up and wave and shout to applause from the rest of the assembly.

London Citizens traces its roots to the Industrial Areas Foundation, the community organisation Obama worked with in Chicago, and the methods it uses to galvanize broad-based community support and participation clearly owe a lot to US churches and citizen groups, along with the use of high-tempo singing and dancing.

The entertainment never distracted from the central purpose of the event, however, which was to get politicians and corporates to sign up to the proposals democratically decided by the organisation's members. If anything it played an important role in the strategy for achieving this by overwhelming them with sheer energy and numbers.

Boris Johnson was brought on stage to give a 'progress report' following promises he'd made at the London Citizens mayoral hustings in April last year. Johnson renewed his commitment to the London Living Wage campaign reporting that City Hall workers now receive the minimum pay of 7.60 an hour it recommends; and after moving personal testimonies on the pain of social exclusion by immigrants from Somalia, China, Uganda, Zimbabwe and Albania, he restated his support for an amnesty for illegal immigrants.

The interactions with politicians and corporates each followed this same 'brilliant and ruthless' formula, as Johnson put it. After hearing testimony from people affected by an injustice, they were put on the spot and interrogated by a leaders to find out where they stand on the organisation's proposed remedies. They squirmed and fudged but nearly all agreed to 'work with' London Citizens and when they did their interrogator would walk across the stage to shake their hand for a photo op ensuring this public commitment was put on record, often with some cute-looking kids jumping into shot for good measure.

The three main parties were asked to respond to London Citizens five proposals to help the poor cope in the recession. Greg Hands of the Conservatives said they would introduce a cap on interest rates for store cards (though not for regular bank cards) and said they were 'open' to statutory regulation of lending; Vince Cable said he accepted all five of the proposals and said that he would 'personally fight for these principles in the Lib Dem manifesto'; and Stephen Timms from the Treasury agreed to meet with London Citizens and the Office of Fair Trading to discuss capping interest rates.

Representatives from the corporate sector were also grilled. Andrew Altman, CEO of Legacy, the company over-seeing the Olympics, agreed to meet quarterly with London Citizens to discuss their goal of a community land trust on the the site of the London Olympics; and, following shocking testimony of the damage inflicted by the predatory lending habits of LloydsTSB, Peter Tyler, of the British Bankers Association, agreed a regulatory framework was needed and that it 'could' be put on a statutory basis.

What lessons, if any, does this campaigning model hold for the wider left? A lot can be learnt from London Citizens and their focus on the grassroots - though this carries with it important reservations.

First, never under-estimate the power of a well-attended public meeting. Online campaigns can generate thousands of signatures in a matter of hours but the barrier to participation is so low it's easy for them to be dismissed and they lack the long-term traction - the delicate mix of persuasion and coercion - which comes from having bums on seats. Clearly, building this level of participation requires slow and pain-staking work but to politicians it represents a powerful electoral bloc and corporates will be desperate to turn up to project a positive image.

Second, be internally democratic. The talk at London Citizens of 'leaders' can invoke images of regimental hierarchy but it's through organising that the campaign goals are decided democratically over many months of one-to-one meetings and assemblies with members. Not only is this right in principle, it gives their demands added weight and legitimacy and means activists have a sense of ownership of campaigns and are more likely to turn up and agitate.

I've seen nothing in this country that carried the same feeling of broad-based popular mobilisation, certainly not with that level of focus and shared purpose. The early anti-war protests were equally diverse and turned out bigger numbers but they never delivered the same sense of the people confronting power and holding it to account.

Part of the story here concerns the creation of the London Mayoralty, as part of a wider programme of devolution which included Scotland and Wales, and the space this has opened up for new forms of accountability and pan-London organisation.

London Citizens pre-dates these reforms but its relationship with the Mayor is clearly central to its work. This demonstrates that, as well as social and economic goals, there is a need to focus on the forms of power that exist and making them more democratic and accountable (something which POWER2010 is attempting to do). You can turn out thousands to march past Parliament against a war, or try and pin down your local MP, but if the institution is suborned and its members gutless, you're unlikely to get very far.

The reservation I have comes from the fact members of London Citizens nearly all come from faith groups or ethnicity-based groups who possess a high degree of social homogeniety and therefore a shared sense of identity and interest. As others have pointed out this is a fantastically effective way of mobilising those groups who already possess high levels of self-organisation, commitment and self-consciousness, to campaign on social issues but there must be serious doubts about whether it can work for those of us (the majority) who aren't members of such communities.

This isn't to detract at all from what I saw Wednesday night, however, which was one of the most diverse, highly focussed and energetic political gatherings I've yet seen.