Police raided two properties in Brighton, on the south coast, taking seven people into custody, a spokeswoman for the Metropolitan Police said, providing no other details about the arrests.
In northern Italy, police took a brother of one main suspect into custody for questioning on Sunday, the Italian news agency ANSA said, but he was not accused of terrorism, ANSA said.
The arrests come as fears were mounting that another wave of attacks was being planned.
A British newspaper said another cell was planning follow-up attacks to the 7 July blasts that killed 56 people, including the four bombers, and the botched 21 July bombings.
One newspaper said more
attacks could hit London
A member of the alleged 21 July cell, fighting extradition from Rome, denied he had links to Osama bin Laden's al-Qaida group and said the attacks were "just a gesture".
At least three men wanted over the attacks were being interrogated by police in London.
Meanwhile, Zambia has decided to hand over another suspect to British officials. Haroon Aswat is a British national who was arrested in Zambia on 20 July.
Zambian officials heeded calls by Britain to hand him over and decided to turn down a US request for his extradition, a police official told AFP on Sunday.
"We have tentatively agreed to hand him over to the British government," said the official, who asked not to be named.
Police guard Lusaka's central prison
where a UK bomb suspect is held
"The US government also requested us to hand him over to them but after further discussions amongst ourselves, we realised that it would be proper that he be handed over to the British government because he is a British national," he said.
The Sunday Times said a third terror cell was planning bomb attacks on London.
The weekly said the group was on the loose and planning further blasts on "soft" targets such as the Underground subway network.
The newspaper said intelligence of a third cell with access to explosives and plans to attack London was known to British police, who have not retracted from Friday's statement that "the threat remains and is very real".
The group was said to be independent of the previous bombers, but members were associated with some of the suspects arrested over the 21 July attacks.
An Eritrean-born suspect was
detained in a west London raid
A Metropolitan Police firearms officer told the newspaper that the arrests so far were "just the tip of the iceberg".
"There is some big stuff coming in the next few months. There's a big network that's got to be cracked."
Suspect bomber Osman Hussain has begun making revelations about the 21 July incidents.
The alleged attempted bomber of an Underground train at Shepherd's Bush station, in custody in Rome, denied links with al-Qaida, the Italian news agency ANSA reported, quoting records of the interrogation.
"I have no connection with al-Qaida," Hussain - also known as Isaac Hamdi - told Italian magistrates after his arrest.
"I joined a muscle-building class in Notting Hill (west London) and I was told we had to act to avenge our people who had been arrested, thrown into prison after the 7 July bombings," the Ethiopian-born naturalised Briton said.
"We did not intend to kill anybody. It was just a gesture"
"A signal had to be given and we did it, but we did not intend to kill anybody. It was just a gesture," he said, adding that he had no links with the four 7 July bombers.
Hussain said the gang used fertilisers and acid, some of which leaked out and wounded his leg.
Hussain is battling extradition to Britain; but the Italian Justice Ministry said documents for his extradition were expected from British police on Monday.
British detectives were continuing to grill other 21 July suspects at the high-security Paddington Green police station.
Eritrean-born Muktar Said Ibrahim, 27, is accused of trying to blow up a Number 26 double-decker bus, and Ramzi Mohammed is suspected of the bungled bombing at Oval Underground train station.
Another suspected cell member, Somali-born Yassin Hassan Omar, 24, suspected of attempting to blow up a subway train at Warren Street station, is reportedly cooperating with police.
A British Sunday newspaper revealed a bag feared to have been meant to aid more bombers enter and exit Britain was found abandoned at London's main Heathrow airport, stuffed with fake passports, documents and bank cards.
Police found a bag with fake
passports at Heathrow airport
The 19 passports inside were for Pakistani, British, Indian, Nepalese and South African nationals.
The holdall also contained a letter written to a Muslim in Dewsbury, northern England - home to 7 July bomber Mohammed Sidique Khan.
Zambian police confirmed on Saturday that Aswat was arrested on 20 July in Lusaka. The British Foreign Office has said it is seeking access to a man being held by the Zambian authorities.
"His arrest follows ongoing security investigations on the threat of terrorism to which Mr Aswat is alleged to be connected," Zambian police Inspector General Ephraim Maateyo said in a statement.
"He entered the country on 6 July. He is a British national and is currently in custody of Zambian security authorities."
A total of 11 suspects were still in British custody over the London bombings and attempted attacks.
Twenty-eight people had been arrested altogether by British police in the terror investigations, the Metropolitan Police revealed on Saturday.
Reports said the girlfriends of 21 July bombing suspects Ibrahim and Mohammed were the two women arrested on Friday fleeing a bag check at London's Liverpool Street railway terminus.
British newspapers said the pair were heading for London's Stansted airport.
The Sunday Mirror quoted a security source as saying the two, released without charge on Friday, were being treated as witnesses.