LAHORE, Pakistan ? Authorities mounted a massive security operation today to hold opposition leader Benazir Bhutto under house arrest for the second time in five days and prevent her from staging a 185-mile protest march against emergency rule.

An aide to Bhutto said her supporters would sweep away the barricades and allow her to embark on the planned three-day procession. However, police swiftly detained the first demonstrators to arrive at the cordon around her residence.

The clampdown intensified the political crisis engulfing Pakistan and further clouded the prospect of a pro-U.S. alliance against rising Islamic extremism forming between Bhutto and President Gen. Pervez Musharraf.

Police detained about 20 Bhutto supporters, including two party officials and two lawmakers, who tried to cross barricades and drove them away in prison service vans.

“They are depriving us of our fundamental right to protest against authoritarian rule and hold a long march for the revival of democracy,” Yusuf Raza Gilani, a former speaker of Pakistan’s National Assembly told reporters as he was led away.

Farzana Raja, a spokeswoman for Bhutto’s Pakistan People’s Party, claimed thousands of its activists had been rounded up to thwart the march. Raja too was detained.

Bhutto’s aide, Sen. Safdar Abbasi, had said the seven-day detention order was not binding because neither Bhutto nor one of her representatives had been served with the document.

However, 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.

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

Thousands of police in riot gear blocked all roads leading to an upscale area of Lahore from where Bhutto wanted to lead the procession. A total of eight trucks or tractors pulling trailers, all of them loaded with sand, were parked across one street early today.

Police stood behind the vehicles and a row of metal barricades topped with barbed wire. The house of a lawmaker where Bhutto was staying was out of sight for reporters, who were prevented from crossing the cordon.

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

White House spokeswoman Dana Perino said that President Bush thinks emergency rule must be lifted “in order to have free and fair elections.”