Police have stepped up patrols around Washington's land-marks, a day after declaring a crime emergency following the murder of a Briton and a spate of other vicious attacks.

Jewish activist Alan Senitt, aged 27, who was working as a volunteer for the potential presidential campaign of former Virginia governor Mark Warner, had his throat slashed on Sunday in the affluent Georgetown area.

Mr Senitt, who was a grad-uate of Birmingham University, was attacked as he and a female companion were returning from a cinema trip. Police say his attackers tried to rape the woman. Four people, including a boy of 15, have been charged with murder.

At least 14 people have been killed in Washington already this month, and in the last 30 days robberies have risen 14 per cent and armed assaults have jumped 18 per cent. Last year, murders in the city fell to a 20-year low of 195.

Police Chief Charles Ramsey's emergency declaration allows commanders more flexibility to adjust officers' schedules and reassign them to high-crime areas.

"That's a short-term solution," Mr Ramsey said. "Long-term is a lot more complicated and it's going to take a whole lot more than the police."

A disturbing aspect to the increase in crime, he said was that "more and more people are being arrested in neighbourhoods they do not live in".

Only hours after the declaration, two groups of tourists were robbed at gunpoint on the National Mall, both by men dressed in black. The US Park Police, who patrol the Mall and are separate from the DC police, posted more officers in the area.

"We're going to reallocate our resources. We're going to see what improvements we can make," said Park Police Sgt Scott Fear.

Park Police are looking for connections between the robberies and three similar incidents in the area in late May.