Security forces said they would continue searching Islamic schools, or madrassas, and other sites thought to be spreading extremism, while President Pervez Musharraf was due to address the nation on the issue later on Thursday.
Security officials said they were holding 228 suspects as of early Thursday.
They said they were following leads that some of the London bombers had met al-Qaida members during trips to Pakistan but denied media reports that a major al-Qaida figure linked to the London attacks had been arrested.
A senior Pakistani security official, speaking on condition of anonymity, said that no significant arrests had been made in connection with the London bombings that killed 52 people and wounded 700.
"Our agencies have questioned a number of people in connection with the London bombing probe in the past 10 days, but so far there has not been any major breakthrough," the source said.
"No significant arrests have been made, and we are not certain whom the British bombers met in Pakistan."
Pakistan has moved to the centre of the investigation into the 7 July blasts after it emerged that three of the bombers, Britons of Pakistani origin, had recently visited Pakistan and that at least one attended a madrassa.
"No significant arrests have been made, and we are not certain whom the British bombers met in Pakistan"
Pakistani security official
Pakistani immigration authorities say Shehzad Tanweer, 22, Mohammed Sidique Khan, 30, and Hasib Hussain, 18, all entered the country recently. The fourth suspect was a 19-year-old Muslim convert born in Jamaica, Germaine Lindsay.
The Financial Times in London on Thursday quoted European intelligence sources as saying: "It is clear that Pakistan has become a key target of the investigation into the London bombings."
The Pakistani security official denied media reports that Briton Haroon Rashid Aswad, a suspected al-Qaida figure linked to the 7 July attacks and seen as the possible mastermind, was among those arrested.
"It has not been established that Haroon Rasheed travelled to Pakistan," the security source said.
British security services had only informed their Pakistani counterparts of phone records between a Pakistani businessman and the suspect's father in Britain, not with proof the suspect had visited Pakistan.
Hussain and two of the other
bombers travelled to Pakistan
One of the leads that security officials were still chasing, the source said, was that both Tanweer and Khan went to Raiwind near Lahore, where they probably met some Arab fighters.
A source close to extremist groups, meanwhile, spoke on condition of anonymity that the three Britons had been in contact with a senior al-Qaida figure hiding in Pakistan.
The al-Qaida man, an Iraqi national, was on the most wanted list of the US Central Intelligence Agency, with a reward of $5 million on his head, the source said.
The dramatic raids in Pakistan came in a week when British Prime Minister Tony Blair and US Secretary of State Condoleezza Rice spoke with Musharraf, a top Western ally since the 11 September 2001 attacks on the US and the US-led invasion of Afghanistan in 2001.
Musharraf will address Pakistan
on new preventative measures
Musharraf, who has escaped at least two assassination attempts, was to address the nation later on Thursday to outline new measures to stop extremist preaching at Islamic schools.
A six-party alliance of Islamic groups, the Muttahida Majlis-e-Amal, called for a nationwide day of protest on Friday, the Muslim day of prayer, "to condemn the global conspiracy against Islam".
"[President] General Pervez Musharraf has resumed the crackdown on religious seminaries and arrests of Islamic scholars and students to please Washington and London," the alliance said.
A leading human rights group cautioned the government not to abuse peoples' rights in the name of curbing extremism as it moved against media outlets said to be preaching hatred.
"The government should proceed according to the law in the crackdown," Asma Jehangir of the private Human Rights Commission of Pakistan said.