Raids were continuing, and police and security agencies were on the lookout for more suspects across all four provinces and in the Pakistan-administered part of Kashmir, officials said on Wednesday.
Of those arrested, 295 allegedly belonged to groups banned by President Pervez Musharraf in the past three years.
"They have been held under the Anti-Terrorism Act, which gives police authority to keep them under detention for a year without indicting them before a court," a senior Interior Ministry official said.
The remaining 300 detainees included Muslim scholars, mosque prayer leaders and others taken into custody for allegedly inciting anti-Western and sectarian hatred through sermons and provocative literature, he said.
"The campaign against militancy and extremism is ongoing, and the police are on alert to nab elements promoting extremism and violence," the official said, speaking on condition of anonymity.
Police launched the raids after British Prime Minister Tony Blair urged Pakistan to move against what he called radical madrassas, or Islamic schools, after news that three of the London bombers - Britons of Pakistani descent - had recently visited Pakistan.
London links denied
Pakistani officials have denied the crackdown is linked to the attacks in London, which killed 52 people and the four bombers.
Musharraf has come under pressure from Muslim groups, including the six-party alliance Muttahida Majlis-e-Amal (MMA), which called the crackdown "state terrorism" and accused Musharraf of being a Western puppet.
The MMA leadership was meeting on Wednesday and expected to call further protests after organising nationwide demonstrations last Friday.
The six-party alliance held
nationwide protests on Friday
The religious right has accused Musharraf of staging the raids as a public relations exercise to please Western allies.
A group representing religious schools said on Wednesday the country would face "dire consequences" unless the crackdown ends.
Madrassas seek evidence
"We demand that the crackdown (against madrassas) end at once," the leader of an alliance of Pakistani Islamic schools, Munibur Rehman, told a news conference.
Rehman said the alliance would cooperate with the government if evidence was provided that a madrassa was involved in extremism.
Madrassas offer free religious education and board for more than one million Pakistani children, especially in areas neglected by state education services.
Thousands of schools were set up, with US and Saudi funding during the eighties.
Musharraf, a key US ally in the region has repeatedly vowed to curb extremism in Pakistan, which also gave logistic support to the 2001 US-led invasion of Afghanistan.
Since assuming power in a bloodless coup in October 1999, Musharraf says he has tried to curb "extremism" by banning 10 organisations.