LAHORE, Pakistan — An opposition leader, Benazir Bhutto, was placed under house arrest for the second time in a week to prevent her staging a march today to protest emergency rule, police said. A close aide to the former prime minister said she would try to lead the 185-mile procession anyway.

The showdown intensified the political crisis engulfing Pakistan and further clouded the prospect of a pro-American alliance against rising Islamic extremism forming between Ms. Bhutto and President Musharraf.

Ms. Bhutto’s aide, Senator Safdar Abbasi, said the seven-day detention order was not binding because neither Ms. Bhutto nor one of her representatives had been served with the document. “We will go ahead with the march,” he told the Associated Press.

Aftab Cheema, the chief of operations of Lahore city police, told the AP that a Bhutto representative had received the order issued by the government of Punjab province, where Ms. Bhutto has been staying at the house of a lawmaker from her party.

“She has been detained and she won’t be allowed to come out,” Mr. Cheema said.

He said about 600 police had been deployed around the house in Lahore and additional forces could be sent later today, when Ms. Bhutto was due to commence her procession to the capital, Islamabad.

A series of three steel-and-barbed wire barricades were erected around the house yesterday and sharpshooters took up positions on surrounding rooftops.

The protest caravan was intended meant to pressure General Musharraf to end the state of emergency he imposed on November 3 and give up his post as army chief. It had been expected to take about three days, and Ms. Bhutto’s party said thousands of supporters were expected to join en route.