LONDON (AP) _ Police arrested a suspected IRA operative at gunpoint Wednesday as he walked down a London street carrying a bomb in a gym bag, Scotland Yard said.

The man, whose name was not given, was taken to a high-security police station, where under Britain's anti-terrorism laws he could remain for up to seven days without being charged or released.

A police spokesman said on condition of anonymity that the bomb contained Semtex, an explosive that has been widely used by the Irish Republican Army.

The arrest seemed likely to boost the image of police, who have often been criticized as ineffective against a well-orchestrated bombing campaign.

In recent years the IRA has intensified its attacks on targets in England as part of its 22-year-old campaign against British rule of Northern Ireland.

Using small bombs and hoax calls, the group has frequently paralyzed London's public transport networks. It has devastated the city's financial district twice with huge vehicle-borne bombs, most recently in April in an attack that left one dead, 45 injured and 157 buildings damaged.

Passersby in Cricklewood, a northwest London neighborhood that is a hub for the city's considerable Irish community, told reporters they saw plainclothes police spring from a black taxi with submachine guns drawn and order the man to the ground as he walked toward a bus stop.

Such an action is especially dramatic in Britain, where many police do not carry firearms.

Scotland Yard refused to comment late Wednesday about how police came by their information, but confirmed that the MI5 intelligence agency took part.

The arrest suggested that police have exploited inside information on IRA operations in Britain.

In the narrow confines of Northern Ireland, police and army intelligence officers rely heavily on informers to catch IRA members in the act. But police efforts to bring suspected members to justice for bombings and shootings in Britain have often embarrassingly misfired.

Last week the police released John Matthews, a 22-year-old Northern Irish student, who had been held for 10 weeks in connection with the attempted bombing of Prime Minister John Major's official London residence in April.

Prosecutors had to admit they had no admissible evidence against Matthews - who, though cleared of any wrongdoing, was then immediately ''expelled'' from Britain back to Northern Ireland under a controversial clause of anti- terrorist law.