Pride's Purge, 'the Rump' and regicide
After Charles I's surrender the Army and the Presbyterians spent many fruitless months trying to negotiate a settlement with him. At the same time he was encouraging uprisings in England and Wales and an invasion from Scotland.
The Army repressed this series of royalist insurgencies in 1648 in the second Civil War, but afterwards its leaders demanded an end to negotiations with Charles, whom they considered a "man of blood" responsible for waging war on his own people.
On 6 December 1648 Colonel Thomas Pride and his soldiers stood outside the entrance to St Stephen's Chapel and, as the Commons convened that morning, arrested 45 Members and excluded a further 186 whom the Army thought were unlikely to support its goal of punishing the King.
After this military coup a further 86 Members left in protest. Pride's Purge left a 'Rump' (as it came to be called) of barely 200 Members. Among these, a determined clique unilaterally forced through an 'Act' on 6 January 1649, establishing a court to try Charles I for high treason - ignoring the negative vote a few days before of the small number of peers still sitting in the Lords.
During the trial in Westminster Hall Charles I disputed the authority of the court and refused to enter a plea. Regardless of the widespread opposition to the trial, a verdict of guilty was pushed through. The death warrant was signed by only 57 of the 159 commissioners of the high court originally established by the Rump, and on 30 January 1649 King Charles I was beheaded outside the Banqueting House on Whitehall.
England becomes a Republic
In March the Rump passed Acts abolishing the monarchy and the House of Lords and in May it passed another Act declaring "the people of England" a "Commonwealth and Free State by the Supreme Authority of this Nation, the Representatives of the People in Parliament ... and that without any King or House of Lords". The parliamentary trinity of King, Lords and Commons had been broken and now the Rump declared full sovereignty over the country.