Security forces raided private houses, mosques and madrassas (schools), across Pakistan and rounded up close to 300 suspects over the past two days in a crackdown ordered by President Pervez Musharraf to curb religious extremism.
"We condemn the London blasts but the countrywide crackdown on mosques and madrassas is absolutely unjustified," said Liaquat Balouch, deputy secretary general of Muttahida Majlis-e-Amal, the main Islamist political alliance.
"We will observe protest day tomorrow."
The crackdown came in the wake of revelations that three of the four suspects in the London bombings were young British Muslims of Pakistani descent.
Intelligence officials say the suspects had visited Pakistan and claimed they had links with hardline groups and some madrassas.
Key US ally
Musharraf, a key US ally in the region, is scheduled to give a televised speech at 8pm (1500 GMT) on Thursday to explain reasons for the crackdown.
Balouch said Musharraf, who seized power in a bloodless military coup in 1999, was using the London bombings as an excuse to promote secularism in the country.
"We condemn the London blasts but the countrywide crackdown on mosques and madrassas is absolutely unjustified"
deputy secretary general of Muttahida Majlis-e-Amal
"The action against mosques and madrassas is part of policy of 'enlightened moderation' to rob Pakistan of its Islamic identity," he said, referring to a term coined by Musharraf to explain his vision of turning Pakistan into a "moderate and progressive" Muslim country.
Muslim groups are opposed to Musharraf's support for the US-led war on terrorism and have been unable to draw big crowds onto the streets in protests against the president.