More than 300 suspected militants have been detained across Pakistan since revelations that three of the four London bombers were British Muslims of Pakistani origin who had visited the country before the attacks.
Pakistan's main alliance of Islamist parties, the Muttahida Majlis-e-Amal, called for protest rallies after Friday prayers, when tens of millions of Pakistanis visit mosques.
But like previous calls for demonstrations against President Pervez Musharraf's support for the US-led "war on terror", this, too, failed to draw big crowds.
Up to 700 Islamists chanted anti-Musharraf and anti-US slogans at Islamabad's Lal (Red) Mosque, which was raided on Tuesday by security forces searching for militants.
Some shouted slogans in support of al-Qaida leader Osama bin Laden and Afghanistan's Taliban government, which was overthrown by US-led forces after the al-Qaida attacks on US cities on 11 September 2001.
Musharraf said all madrasas
must be registered
The protesters pelted stones at a police post, destroyed lamp posts and set fire to a police motorcycle.
Similar rallies were held in the cities of Karachi, Lahore, Quetta and Peshawar. Many of the protesters were students from Islamic schools, or madrasas.
The protests followed a televised address to the nation by
Musharraf on Thursday night in which he called for a holy war
against preachers of hate and announced steps to rein in
militant madrasas and groups seen as having influenced the
Mairaj-ul-Huda, an MMA leader in Karachi, questioned why there should have been a crackdown in Pakistan.
"British nationals are involved in the London blasts," he told a rally of about 600 supporters in Karachi. "Why then is there a crackdown on religious institutions and religious scholars in Pakistan?"
In his television address, Musharraf said all madrasas must
register with authorities by December.
In a rare show of solidarity, self-exiled former prime minister Benazir Bhutto, for long Musharraf's bitter rival, backed his decision to register madrasas.