From the beginning of his reign, King Charles I's relations with the Parliament of England were strained. In 1625, he married a Catholic, Henrietta Maria, daughter of Henry IV of France: an unpopular move with many of his subjects. The next year he dismissed Parliament after MPs tried to have his chief minister, George Villiers, removed from office - only to recall it later when he needed money for his war against France and Spain. In 1628, Parliament drew up a Petition of Right detailing their grievances with the King, which included regular Parliaments and their approval of any new taxes. The next year, Charles had an MP arrested for not paying a tax that had not been approved by MPs and then dissolved Parliament declaring his divine right to rule.
Charles then ruled the country for the next eleven years without calling a Parliament. In this time he imposed a tax called Ship Money on the whole country, which previously only the residents of coastal towns paid to fund the Royal Navy. Nevertheless, even with this extra income Charles did not have sufficient funds to finance a campaign against rebellious Scots. As a result he finally recalled Parliament in 1640.
In return for new taxes, the Commons made a number of demands, including an end to the collection of Ship Money. The King responded by dismissing them after only a few weeks, but relented and recalled them later that year. In the summer of 1641 another rebellion broke out; this time in Ireland. Parliament criticised the King's ability to control the Scots and Irish and suggested that they be responsible for the security of the realm rather than him.
In 1642 matters reached a head. The King issued warrants for the arrest of one peer and five members of the House of Commons on charges of treason. When Parliament refused to accept the charge, Charles ordered troops to the Commons to arrest the accused, who, forewarned, had already fled. The King then sent his wife to the Catholic monarchs of Europe to request military aid against the parliamentarians, who responded by taking control of the militias. Finally, on 22nd August 1642, King Charles I raised his standard at Oxford, declaring war on Parliament.
Civil war raged for nearly four years before the King finally surrendered on 6th May 1646 to the Scottish army (which fought on the side of Parliament). He managed to escape from custody the next year, but his intransigence had sealed his fate: following a purge of Parliament to remove any last vestiges of support for the King, he was tried and found guilty. On 30th January 1649 at Whitehall Palace, he was executed.
Those Members of Parliament that remained following the purge - known as the Rump Parliament - started to create the legislative framework and institutions to rule without a monarch. This culminated when they passed An Act declaring England to be a Commonwealth on 19th May 1649. The text of which is as follows:
Be it declared and enacted by this present Parliament and by the Authoritie of the same That the People of England and of all the Dominions and Territoryes thereunto belonging are and shall be and are hereby constituted, made, established, and confirmed to be a Commonwealth and free State And shall from henceforth be Governed as a Commonwealth and Free State by the supreame Authoritie of this Nation, the Representatives of the People in Parliament and by such as they shall appoint and constitute as Officers and Ministers under them for the good of the People and that without any King or House of Lords.
Britain continued without a monarch for the next eleven years until the Restoration of Charles II in 1660 ended what had effectively become a military dictatorship. Nevertheless, Parliament had asserted its power and set a precedent of removing an individual from the Throne. England was never going to be an absolutist monarchy.
See David Plant's excellent British Civil Wars site for more details of the Civil Wars, Commonwealth and Protectorate.