LONDON (AP) _ Police warned Londoners to be alert for bombs after an attack on a gay pub in central London that killed two people, the third bombing aimed at minorities in less than three weeks.

The explosion in the Admiral Duncan pub in the heart of Soho, the capital's restaurant district, injured at least 70 people Friday evening. Twenty people remained hospitalized today, with at least four in critical condition.

``This is a time for calm. It is a time for vigilance. It is a time for the community working together to defeat these cowards,'' London Police Commissioner Paul Condon said.

Police said they believed the blast was linked to two nail bombs that injured dozens in minority neighborhoods in the past two weeks.

Soho is always busy on Friday nights, but this week thousands of diners and drinkers were enjoying the balmy start of a three-day weekend.

Many were maimed, and scores of injured filled the glass-strewn streets as sirens wailed and ambulances threaded their way through rush-hour traffic trying to reach the scene, which adjoins Chinatown and the theater district.

The White Wolves, a racist group that has threatened black legislators, claimed responsibility for the attack in a phone call to the British Broadcasting Corp. two hours after the blast.

Combat 18, a right-wing extremist group, claimed the two previous bombs _ one on April 17 that injured 39 in mostly Afro-Caribbean neighborhood of Brixton, and another on April 24 that injured seven in the heavily Bangladeshi district of Brick Lane.

``Clearly, they are linked. Clearly, they are hate devices,'' Condon said. ``It is an attack on us all and we must work together to catch these people as soon as possible.''

Since the attackers appear to be making their way through London's minorities, the Jewish community acknowledged fears.

``We are concerned, but I hope that the precautions we have put into effect will stop anything happening,'' said Mike Whine of the British Board of Jewish Deputies.

The police did not explicitly confirm Friday's blast was caused by a bomb, but Alan Fry, head of Scotland Yard's Anti-Terrorist Branch, said ``there are injuries sustained by people tonight that indicates that it was a nail bomb.''

The force of the blast blew out the front of the pub. The scene inside the pub was ``surreal,'' said Ken Murphy, a paramedic who arrived at the scene within four minutes.

``There was a lot of moaning, a lot of people shouting to me. They were groaning, lying on the floor,'' he said. ``People were calm. They knew what had happened straight away.''

The two dead had suffered ``massive injuries,'' he said.

``There were certain people really badly injured and I just tried to keep them talking until the police arrived,'' said witness Tony Howard, 35. ``The injuries were horrific. There were bones sticking out of legs, people's faces were burnt.''

Police and other organizations have offered a $840,000 reward for the arrest of the bombers in the two incidents.

On Friday, a day after issuing a video of their prime suspect, police said they had arrested two men in connection with the Brixton attack. Neither was the young white suspect seen in video footage released Thursday, but police would divulge no other details about the arrests.

Police arrested a third person today, Scotland Yard said. The spokesman, speaking with customary anonymity, refused to provide any other information.

Several black lawmakers reported receiving threatening letters signed by the ``White Wolves.''

London's Sunday Telegraph reported that a 15-page document signed ``White Wolves'' and giving instructions on orchestrating a bombing campaign was faxed to a radio station a week before the Brixton bombing.

One of Britain's largest Asian community newspapers also received a threatening letter from the ``White Wolves,'' the paper's editor said Sunday.