On 20 January, Charles was charged with high treason 'against the realm of England'. Charles refused to plead, saying that he did not recognise the legality of the High Court (it had been established by a Commons purged of dissent, and without the House of Lords - nor had the Commons ever acted as a judicature).
The King was sentenced to death on 27 January. Three days later, Charles was beheaded on a scaffold outside the Banqueting House in Whitehall, London.
The King asked for warm clothing before his execution: 'the season is so sharp as probably may make me shake, which some observers may imagine proceeds from fear. I would have no such imputation.'
On the scaffold, he repeated his case: 'I must tell you that the liberty and freedom [of the people] consists in having of Government, those laws by which their life and their goods may be most their own. It is not for having share in Government, Sir, that is nothing pertaining to them. A subject and a sovereign are clean different things. If I would have given way to an arbitrary way, for to have all laws changed according to the Power of the Sword, I needed not to have come here, and therefore I tell you ... that I am the martyr of the people.'
His final words were 'I go from a corruptible to an incorruptible Crown, where no disturbance can be.'
The King was buried on 9 February at Windsor, rather than Westminster Abbey, to avoid public disorder. To avoid the automatic succession of Charles I's son Charles, an Act was passed on 30 January forbidding the proclaiming of another monarch. On 7 February 1649, the office of King was formally abolished.
Peace be with you