Politics and much of national life came to a halt as the unprecedented scale of the outrage in the US unfolded.

The Conservative party suspended counting of votes in its leadership contest for at least 24 hours. The result, which had been due to be announced with a fanfare at a London event at 5.10pm today, will be issued in a simple statement tomorrow.

The prime minister abandoned his planned speech at the Trades Union Congress in Brighton and returned to Downing Street.

Mr Blair, his voice cracking with emotion, sent his "deepest condolences" to President George W. Bush and said: "We can only imagine the terror and the carnage there and the many, many innocent people who will have lost their lives."

He called on democracies to come together to fight "the new evil" of mass terrorism as Britain offered whatever help it could to bring to justice the perpetrators of the attacks in the US.

"This mass terrorism is the new evil in our world today. It is perpetrated by fanatics who are utterly indifferent to the sanctity of human life. We, the democracies of this world, are going to have to come together to fight it together and eradicate this evil completely from our world."

Jack Straw, foreign secretary, said government buildings, official and military, had been raised to a higher state of alert. Alert states for diplomatic posts overseas were raised to the highest level.

He issued travel advice to British citizens to take great care if they were living overseas in sensitive areas, stay in their compounds, avoid obvious areas of potential risk and have great care for their safety.

Mr Straw said: "Of course we are a very close ally of the United States, we work closely with them, both militarily and in terms of intelligence, and we will do everything that we can to assist the US in bringing these people to justice. We also have to do everything we can in the hope of averting further terrorist incidents."

The Queen expressed her "growing disbelief and total shock" as devastation unfolded in the US. In a message to Mr Bush, she expressed "heartfelt sympathy" and said her thoughts and prayers were with Americans.

William Hague, the Conservative leader, described the events in America as a "monstrous act of war against the civilised world" and sent out his sympathies to the families of those killed or injured.

Charles Kennedy, the Liberal Democrat leader, said: "We offer our profound sympathies to the US public and to the families of the countless numbers that must have been killed and injured in this blatant set of attacks.

"The international democratic community must come together as never before to see down the forces of sheer evil that perpetrate such crimes."

Mr Blair, who had expected a frosty reception from the union audience following a row over policy on public services, received a standing ovation as he left the stage.

The TUC suspended the session of its conference and cancelled the annual dinner. Most ministers in Brighton returned to London.