May Day ("May Fayre") has been synonomous with "a day of protesting" in the UK (and indeed the rest of the world it seems) and has been dubbed as a "spring ritual" by the media. Anti-capitalist rallies are held around the country, the largest being held in London (the square mile of Central London usually). In recent years these protests have been marred by vandalism (causing thousands of pounds worth of damage in Oxford Street, Soho and Parliament Square).

Timeline of protests
    May 1, 2003
    Protesters gathered around Trafalgar Square in a mostly peaceful (only 7 arrests were made compared to 54 in 2002) demonstration and the majority had dispersed by 4pm. Around 200 people remained and danced to tunes provided by a mobile sound system or simply sat and watched. A group marched from Trafalgar Square to Parliament Square where they climbed trees and shouted at Downing Street. The theme was "weapons of mass destruction" and small groups of demonstrators around the country staged protests against arms and oil companies.

    May 1, 2002
    Two seperate protests - the TUC (trade union) march which went from Green Park to Trafalgar Square (with approval from the authorities), and an independent protest by anarchists held around Mayfair. Also worth mentioning was the "Critical Mass bike ride" in the morning from Camden Town to the American Embassy at Grosvenor Square. Trouble flared up when police engaged demonstrators in the Soho area. The police were later sued by a handful of individuals for wrongful detention.

    May 1, 2001
    Central London is paralysed by a day of violence and some shops are attacked by a hardcore of protesters. Police in riot gear detain thousands of peaceful protesters in Oxford Street and nearby Soho for up to eight hours.

    Police evoke special powers to detain the crowd that includes many innocent people who were accidentally caught up in the demonstration.

    May 1, 2000
    Central London descends into chaos as several thousand protestors descend on Whitehall in the first of what will be known as the "May Day Protests". After several hours of peaceful protest a number of protestors leave Parliament Square and march up towards Trafalgar Square. A number of monuments, including Winston Churchill's statue and the Cenotaph are defaced and a branch of McDonald's is vandalised (yay). The police operation - the biggest in the capital for a political demonstration for 30 years - are criticised for taking a 'softly softly' approach. Thirty people are arrested.
    Full coverage...

If you've got any more information, particularly May Day world events, please /msg me.
Some of the timeline information was legally reproduced from Guardian Unlimited (free news service).

Further reading: