London's police chief warned Thursday that more cells of would-be bombers could strike as police made nine arrests in their hunt for the men behind failed attacks on the city's transport system last week.

Commissioner Ian Blair said the capital had been lucky that bombs on three underground trains and a bus had not exploded fully on July 21. Three men still wanted for the attacks remained a danger and might not be the only threat, he said.

"It does remain possible that those at large will strike again. It does also remain possible that there are other cells that are capable and intent on striking again," he told a police authority meeting.

Police arrested nine men in Tooting, south London, Thursday morning, bringing to 20 the number of people being held in connection with the failed July 21 attacks.

Police said the nine did not include the three suspected bombers they are still hunting. They arrested one of the four prime suspects in a dramatic dawn raid on Wednesday.

They hope the arrest of Yasin Hassan Omar, 24, will provide a breakthrough in the search for the three other suspects.

Blair said his force was reviewing 15,000 closed circuit television tapes, had taken 1,800 witness statements and received 5,000 calls to its anti-terrorism hotline.

Omar, who came to Britain from Somalia as a child refugee, was wanted in connection with an attempted attack at London's Warren Street underground station on July 21.

Armed police used a stun gun when they arrested him in a raid on a house in the central English city of Birmingham.

Police were under pressure to exercise caution after they shot dead Brazilian Jean Charles de Menezes in London on Friday after mistaking him for a suicide bomber.

The press also published front-page pictures of a ready-made nail bomb found in the boot of a car that had been rented by one of the July 7 attackers. They showed a bottle studded with nails to act as shrapnel.

Chilling black and white X-ray photographs of the bomb were plastered across the front page of most British newspapers.

Reports that up to 16 bombs were found in the car have been taken as evidence of a widespread terror campaign.

The bombs were found in a rental car abandoned at a railway station in Luton, north of the British capital, by the four suicide bombers, who boarded trains to King's Cross exactly three weeks ago, newspapers reported.

Source: CD/agencies