ISLAMABAD — President Musharraf placed opposition leader Benazir Bhutto under house arrest for a single day today — surrounding her villa with barbed wire — and rounded up thousands of her supporters to block a mass rally against his emergency rule.

The crackdown dimmed hopes the two pro-American leaders could ever form an alliance. A top American official voiced fears it would obstruct the fight against Islamic extremism — a threat underlined by a suicide bombing in northwest Pakistan that targeted a Cabinet minister, who escaped unharmed.

America called for the restrictions to be lifted, and the acting deputy commissioner for Islamabad, Aamir Ali Ahmed, said late today that Ms. Bhutto was again free to move about. But barriers and police remained outside her home.

Party members said she would try to leave tomorrow morning.

Ms. Bhutto tried to leave today by car but was blocked by police after a scuffle with her supporters who tried to remove a barricade. The former prime minister had planned to address a rally in nearby Rawalpindi, defying a ban on public gatherings.

In Rawalpindi, about 200 Bhutto supporters were dispersed by police using tear and batons. Dozens were arrested. There was a second, smaller clash between protesters and police. Aside from those incidents, the streets of the garrison city were mostly deserted.

A top Interior Ministry official, Kamal Shah, said a district magistrate had served a “detention order” on Ms. Bhutto — who last month escaped an assassination attempt by suicide bombers, an attack that killed more than 145 people — so she could not leave her home.

However, speaking by phone from the scene, Ms. Bhutto said that no arrest papers had been served on her.

“If I’m arrested the People’s Party of Pakistan workers will continue to fight for democracy and the rule of law,” she told reporters who heard the call via speakerphone. She said that 5,000 members of her party had already been detained.

An Islamabad police official, Afzal Khan, said that officers blocking Ms. Bhutto’s way were following a government order under which she could not hold the rally. The Rawalpindi mayor said there was a “credible report” that six or seven suicide bombers were preparing to attack it.

The crackdown showed that a week after suspending the constitution and assuming emergency powers, President Musharraf was not letting up on his political rivals despite saying yesterday that parliamentary elections would go ahead by mid-February, just a month later than originally planned. His announcement came after intense pressure from America, his chief international supporter.

Today’s moves will further sour relations with Ms. Bhutto and hurt the prospects of the two pro-Western leaders forming a postelection alliance against religious extremism.

Ms. Bhutto tried to leave for Rawalpindi inside a white Landcruiser with tinted windows, surrounded by about 50 supporters, including several lawmakers, an intelligence official at the scene said.

Other Bhutto supporters pulled at a barbed wire barricade on the street to make way for the vehicle, but were blocked by police, the official said on condition of anonymity as he was not authorized to speak to media. Ms. Bhutto’s vehicle only managed to travel about 40 yards before it was stopped.

“We are trying to pass through because we want to reach Rawalpindi. There was a baton charge. There was a barbed wire. People in Kashmir were also stopped from reaching here. Those who can reach Rawalpindi, they should try to reach there,” Ms. Bhutto later told private Geo TV.

“The government says that some suicide bombers have entered Islamabad. If they have any such information, then why can’t they arrest them?” she said.

She also demanded that General Musharraf step down as army chief by next week when his presidential term expires.

“He should be retired as the chief of army staff by November 15,” she told reporters.

Dozens of police, some in riot gear, were deployed at Ms. Bhutto’s residence by barbed wire and steel and concrete barriers. Other security personnel patrolled on motorcycles, horseback and in armored vehicles. At least 12 Bhutto supporters were arrested, including a woman who showed up with flowers.

In the second clash in Rawalpindi, about two dozen supporters burst out of an alley, shouting, “Long Live Bhutto!” Police, some on horseback, others banging their shields, chased them away. Other supporters set a tire and garbage on fire. Police fired tear gas shells from an armored personnel carrier, and the protesters pelted the police with stones.

In the northwestern city of Peshawar, police used force to disperse about 300 Bhutto supporters trying to reach Rawalpindi. About 25 were arrested.

Ms. Bhutto’s decision to join in anti-government protests against General Musharraf is another blow to the military leader whose popularity has plummeted this year amid growing resentment of military rule and failure by his government to curb increasing violence by Islamic militants.

Critics say that General Musharraf — who seized power in a 1999 coup — declared the emergency and ousted independent-minded judges to maintain his own grip on power. The moves came days before the Supreme Court was expected to rule on whether his recent re-election as president was legal.

General Musharraf said the declaration of emergency last Saturday was needed to put an end to political instability and to fight Taliban and Al Qaeda-linked militants.

But most of the thousands of people rounded up countrywide have been moderates — lawyers and activists from secular opposition parties. Police have squashed attempts by lawyers to protest. Hundreds of students have also stage demonstrations on university campuses.

Across the normally bustling city of Rawalpindi, where the headquarters of Pakistan’s army and General Musharraf’s residence is located, streets were largely deserted, aside from the two clashes with police. Today also marked a public holiday in Pakistan. The road to Islamabad had been blocked by two tractor trailers and a metal gate.

“Since the government has not given permission for it due to security reasons, we will not allow any one to gather here for the rally,” the city’s police chief, Saud Aziz, said.

Police were also on the lookout for potential suicide bombers, who Mr. Aziz warned were preparing a repeat of the October 18 bombing of Ms. Bhutto’s jubilant homecoming procession in the southern city of Karachi after eight years of exile. She escaped unharmed, but more than 145 people died in the attack, blamed on Islamic militants.

Rawalpindi has also been hit by a series of suicide attacks, targeting the military.

Ms. Bhutto’s party said its 5,000 arrested supporters were picked up in the last three days across the eastern province of Punjab.

“It is a massive crackdown on our party,” a PPP lawmaker, Raja Javed Ashraf, said.

A security official, speaking on condition of anonymity because he was not authorized to speak to journalists, said only 1,000 Bhutto supporters had been detained.