ISLAMABAD, Pakistan — President Musharraf yielded to pressure from America yesterday and said Pakistan will hold parliamentary elections by mid-February, just a month later than originally planned.

But the military leader showed no sign of letting up on his political foes, reportedly arresting more than 800 supporters of an opposition leader, Benazir Bhutto, before dawn in an effort to head off a major anti-government demonstration set for Friday.

The White House hailed its ally’s election pledge, but Bhutto denounced his announcement as “vague” and demanded General Musharraf give up his second post as army chief within a week. She said the mass protest would go ahead despite warnings it could be targeted by suicide bombers.

Ms. Bhutto is leader of Pakistan’s biggest party and her decision to join in anti-government protests was another blow for General Musharraf, who has seen his popularity slide this year amid growing resentment of military rule and increasing violence by Islamic terrorists. In a fourth day of protests against the general’s imposition of emergency rule over the weekend, lawyers rallied peacefully in Islamabad, while demonstrators clashed with police in the border city of Peshawar.

General Musharraf has been under increasing pressure to quickly hold elections and resign as army commander since he suspended the constitution Saturday. He said emergency measures were needed to calm political instability he claims is hampering the fight with Taliban and Al Qaeda-linked terrorists.

President Bush, who counts the Pakistani leader as a key ally in the war with extremist groups, personally got involved Wednesday, telling General Musharraf in a phone conversation that “the United States wants you to have the elections as scheduled and take your uniform off.”

Some Pakistani officials had said earlier the election could be delayed by up to a year, but General Musharraf went on state-run TV to announce the ballot would “be held before February 15.” He wore a blue business suit rather than his general’s uniform.

“We think it is a good thing that President Musharraf has clarified the election date for the Pakistani people,” a statement from the Bush administration, which has been pressing him to return the country to civilian government, said. Despite that pressure, Pakistani officials denied General Musharraf’s election decision was made because of the American demands or the spreading anger among Pakistanis frustrated by military rule.

“The government takes decision itself and there is no internal or external pressure on it,” said the ruling party’s president, Chaudhry Shujaat Hussain. Critics contend General Musharraf suspended the constitution, blacked out dozens of TV news channels and ousted independent-minded judges to maintain his own grip on power, which he seized with a 1999 coup. His emergency decree came as the Supreme Court was expected to rule on the legality of his reelection as president last month in a vote by legislators. Opponents say it was unconstitutional for him to be a presidential candidate because he also holds the powerful post of army chief.

In his TV appearance, General Musharraf said he would be sworn in for a new five-year presidential term and resign as army commander once the Supreme Court — now purged of his sharpest critics — validates the vote.

Analysts predicted emergency measures would be lifted very soon after that happens. They noted it would be necessary to ease security restrictions to allow election campaigning, because the emergency rules make it illegal to hold public gatherings.

In defiance of that ban, Ms. Bhutto called for a huge anti-Musharraf demonstration Friday in Rawalpindi, a garrison town on the southern outskirts of the capital, and there were fears it could turn violent.