British police are on high alert to prevent any terrorist attack during the state visit of U.S. President George Bush, which begins Tuesday night. London's mayor has appealed for anti-Bush demonstrators to protest peacefully.

Police in London are mounting a major security operation to protect President Bush, control anti-Bush demonstrations and guard against a terrorist attack.

The deputy superintendent of the Metropolitan Police, Andy Trotter, said thousands of extra officers will be deployed on the streets of London every day during the Bush visit, which coincides with a heightened anti-terrorist alert.

"We are concerned about the general terrorist threat to Britain at the moment, and particularly to the capital, although there is no specific threat against President Bush. We are very concerned about the security level at the moment and obviously that is one of the reasons we have so many police officers on duty for the next four days," said Superintendent Trotter.

London Mayor Ken Livingstone said the police face what he describes as "an almost impossible task." Thousands of protesters plan to demonstrate against President Bush this week, and the mayor is appealing for them to do so peacefully.

"There is no place for violence of any kind in London this week. Protests must be peaceful and they must be within the law. The police will ensure that there is, within sight and sound of George Bush's every appearance, a place designated for lawful protest, but they will be equally determined that there shall be no violence and I urge demonstrators not to cede the high moral ground," he said.

The leftist mayor is highly critical of President Bush and he participated in previous anti-war rallies before the invasion of Iraq. This week, however, Mr. Livingstone says he will stay at his office and monitor the situation in case there is any trouble.