The Gordon Riots (London) were triggered by the proposed Papist Act of 1780. The proposed law would have removed some of the penalties on Catholics that were enacted by the Popery Act of 1698. Lifted from Wikipedia:
March on Parliament
Despite being aware of the possibility of trouble, the authorities had failed to take steps to prevent violence breaking out. The Prime Minister, Lord North, had forgotten to issue an order mobilising the small number of Constables in the area. Those that were present in the House of Commons were not strong enough to take on the angry mob. Eventually a detachment of soldiers were summoned, and they dispersed the crowd without violence. Inside the House of Commons, the petition was overwhelmingly dismissed by a vote of 192 to 6.
Once the mob around parliament had dispersed, it seemed to the government that the worst of the disorder was over. However, the same night a crowd gathered and attacked the Roman Catholic Chapel of the Sardinian Embassy Chapel in Duke Street. Bow Street Runners and soldiers were called out and made thirteen arrests, although most of the ringleaders had managed to escape. The same night the Bavarian Embassy was attacked, and crowds caused random violence in streets known to house rich Catholics.
Map of London Wall, Moorgate, Moorfields and Bethlem Royal Hospital from John Rocque's Map of London, dated 1746.
The area of Moorfields, one of the poorest parts of the city, was the home of many Irish immigrant workers and had a large area of open ground where crowds could assemble. Despite the appeal of a prominent Irish merchant, James Malo, to the Lord Mayor, Brackley Kennett, no additional protection was offered to the area. During 3 June a crowd had gathered in Moorfields, and by nightfall it began to go on the rampage. Malo's house was amongst the many to be sacked and burned.
Newgate Prison was attacked and largely destroyed, as was The Clink. This allowed large numbers of prisoners to escape, many of whom were never recaptured. Severe destruction was inflicted on Catholic churches and homes and chapels on the grounds of several embassies, as well as on the Bank of England, Fleet Prison, and the house of the Lord Chief Justice, William Murray, 1st Earl of Mansfield.
The army was called out on 7 June and given orders to fire upon groups of four or more who refused to disperse. About 285 people were shot dead, with another 200 wounded. Around 450 of the rioters were arrested. Of those arrested, about twenty or thirty were later tried and executed. Gordon was arrested and charged with high treason, but was found not guilty. Brackley Kennett, the Lord Mayor, was convicted of criminal negligence for not reading out the Riot Act and given a £1,000 fine. The military units which dealt with the rioters included the Horse Guards, Foot Guards, the Honourable Artillery Company, line infantry including the Queen's Royal Regiment (West Surrey), and militia brought in from neighbouring counties.