The event was called by the three main Lambeth anti-poll tax groups: Lambeth Against the Poll Tax (associated with SWP and various Labour leftists), Lambeth Anti Poll Tax Unions (associated with Militant, now the Socialist Party) and Community Resistance Against the Poll Tax (more direct action/anarchist oriented). At one level it was absurd that there were three separate groups, but there were political differences - notably the fact that the first two spent a lot of energy calling on Labour councillors not to implement the poll tax, whereas the latter thought that a waste of time. But all called for people to refuse to pay their poll tax, and were able to work together to the extent of calling for actions like this one.
... as well as Poll Tax bills:
At a very local level there were also groups which cut across these divisions. I was in a group called Tulse Hill Estate Against the Poll Tax which included various kinds of radicals (Red Action -including a member of Blaggers ITA, SWP, anarchists etc.). And on that day we marched down from the estate, stopping off by the George Canning (now Hootananny), before going on to Brixton Oval (now known as Windrush Square).
There to burn a Conservative Party flag...
That poll tax leaflet in full (click to enlarge). The key issue with the poll tax - officially known as the Community Charge - was that by making everybody pay the same amount to their local Council, regardless of income or house size, it was effectively shifting more of the burden of local taxes from the wealthier to the poorer.
The campaign against the poll tax was ultimately successful. The goverment was forced to withdraw it and replace it with the Council Tax - not massively better perhaps, but at least reinstating some element of people living in more expensive properties paying more. And soon afterwards the increasingly unpopular Thatcher was forced out in a palace coup by Tory MPs.