Samantha Rigg-David, the sister of Sean Rigg, a 40-year-old man who died in police custody in 2008 in Brixton, and Alastair Morgan, whose brother Daniel - a private detective - was found with an axe in his head in a south London pub's car park in 1987, took part in the demonstration.
Amanda Jacks, from the FSF, said up to 300 people, including Huddersfield Town supporters, are due to take part in the demonstration.
BBC: Football fans to stage protest over police restrictions
The three-week trial of Mr Meadows, and four other young men, heard that more than 10, 000 people took part in the demonstration against government plans to increase tuition fees and the majority behaved "entirely lawfully".
BBC: Alfie Meadows: Student protest jury fails to reach verdict
Cathedral authorities have invited the activists to take part in negotiations to try to end the demonstration.
BBC: Ed Miliband warns of St Paul's protest 'danger signals'
Right-wing National Front supporters staged a demonstration in one part of the town while members of the Anti-Nazi League staged a rival protest in another part.
Mr Peach, a 33-year-old teacher from New Zealand, was taking part in a demonstration against the National Front.
He will limber up at the Kuortane Sports Institute, in western Finland, before performing a demonstration run in his McLaren at London's Wembley Stadium as part of the Race of Champions event on 14 December.
Thousands of people took part in a demonstration last month opposing the removal of children's heart surgery from the hospital.
They saw more than 1, 000 athletes from 44 countries participating and people with quadriplegic spinal injuries competed for the first time while visually impaired athletes took part in demonstration events.
Going forward, the LLCD will play a vital part in NASA's 2017 Laser Communications Relay Demonstration, a key test of laser-based relay comms in future missions.
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