Foreign Secretary David Miliband has urged restraint on all sides in Pakistan after president Pervez Musharraf declared emergency rule.
Foreign Secretary David Miliband made his first UN speech
The move came ahead of a Supreme Court ruling on the president's re-election.
Mr Miliband said he was gravely concerned about the development and said the UK would raise concerns "at the highest levels".
In Pakistan, troops have been deployed inside state-run TV and radio stations and independent channels went off air.
The Supreme Court had been due to rule on the legality of Gen Musharraf's re-election victory in October.
He declared the state of emergency amid rising Islamic militant violence ahead of the judges' decision.
Mr Miliband said: "All friends of Pakistan will be concerned by the turn of events today.
"We recognise the threat to peace and security faced by the country, but its future rests on harnessing the power of democracy and the rule of law to achieve the goals of stability, development and countering terrorism."
The foreign secretary said it is "vital that the government acts in accordance with the constitution, and abides by the commitment to hold free and fair elections".
He added: "We will raise our concerns about these developments at the highest levels."
Mr Miliband said the UK is working closely with "friends of Pakistan" on the international stage in a bid to encourage all parties to show restraint and "work together for a peaceful and democratic resolution".
The foreign secretary also called for UK-based Pakistanis to use family connections to call for democratic rule.