Wikipedia
The Poll Tax Riots were mass disturbances, or riots, in British cities during protests against the Community Charge (commonly known as the poll tax), introduced by the Conservative government led by Prime Minister Margaret Thatcher. By far the largest occurred in central London on Saturday March 31, 1990, shortly before the poll tax was due to come into force in England and Wales. Many believe the riot - the largest in the city in the 20th century - caused Thatcher's downfall eight months later.

The disorder in London arose from a demonstration which began at 11am. The rioting and looting ended at 3am. next morning. This riot is sometimes called the Battle of Trafalgar, particularly by opponents of poll tax, because much of the rioting took place in Trafalgar Square.