Authorities mounted a massive security operation Tuesday 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 a group of Bhutto supporters, including two lawmakers, who tried to cross the 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.

Police said that while up to 30 people were arrested, Gilani was merely taken back to his home.

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.

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 Tuesday.

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.

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.

"She will defy the ban," Abbasi told The Associated Press by telephone. "We are ready for the long march, and our supporters will remove all the police blockades in the way of their leader."

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.

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.

Asked to comment on Bhutto's house arrest ahead of Tuesday's protest, National Security Council spokesman Gordon Johndroe said: "While the situation continues to evolve, we believe that peaceful protests should be permitted and those detained should be allowed to participate."

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

Prior to Bhutto's re-arrest, police said they had ramped up security around her due to intelligence that a suicide bomber was planning to attack her in Lahore. Ayaz Salim, a top police official, said officers had searched all the city's hotels after receiving a tip that a suicide bomber was staying in one, but they did not find the suspect.

Bhutto was targeted by an Oct. 18 suicide bombing attack on a homecoming procession in the southern city of Karachi as she returned from years in exile. The assassination attempt killed 145 other people.

She was first placed under house arrest in Islamabad Friday to prevent her from addressing a rally in the nearby garrison city of Rawalpindi, where authorities also warned they had intelligence that suicide bombers were loose in the area.

With an escort of dozens of police vehicles, Bhutto ventured out around Lahore Monday to offer prayers at the grave of Pakistan's national poet, Allama Iqbal, and declared to reporters that her caravan was part of her campaign "to save Pakistan."

"I know it is dangerous but what alternative is there when the country is in danger?" the former prime minister said.

With Musharraf losing popularity due to growing disaffection in Pakistan over enduring military rule, the Bush administration has backed talks about power-sharing between him and Bhutto as a way to keep a U.S.-friendly administration in control of the nuclear-armed nation where militants are orchestrating attacks inside the country, across the border in Afghanistan.

Foreign Ministry spokesman Mohammed Sadiq said Monday that Pakistan's nuclear weapons were secure and that there was no risk they would be seized by Taliban or al-Qaida-linked militants who have expanded their influence beyond northwestern border regions.

"There are multiple layers of command and control and the weapons are not in danger of falling into any hands," he said. "Pakistan's nuclear program is very well-guarded."

Musharraf has set no time limit on the emergency declaration, which has resulted in the arrests of thousands of his critics, a ban on rallies and the blacking out of independent TV networks. He said Sunday that the emergency was necessary to step up the fight against militants and ensure "absolutely fair and transparent elections" for parliament.

However, the imposition of emergency rule came shortly before the supreme court was due to rule on the legality of his recent election for a new presidential term. As a result, Musharraf has found it hard to shake suspicions it was actually a tactic to oust judges who could have obstructed his bid to extend his eight-year rule.

Bhutto has said the talks with Musharraf stalled because of the emergency declaration but could be revived if he rolls back emergency rule.

She joined other Pakistani opposition leaders in questioning whether a free and fair ballot would be possible under emergency rule. She welcomed Musharraf's commitment to holding elections on time in January, but likened campaigning under the emergency to being tied and blindfolded.

"In the prevailing circumstances you can't say the elections will be free and fair," she told reporters Monday.

"Boycotting elections could be an option," she said. "We will consult the other political parties."

Other opposition parties already have threatened a boycott.

Bhutto also demanded that Musharraf step down as army chief when his current term as president expires Nov. 15 — a step he is promising to take once a reconstituted supreme court validates his recent presidential election victory.

Foreign ministers from the Commonwealth of Britain and its former colonies said that Pakistan would be suspended from the organization unless the state of emergency was repealed and Musharraf stepped down as army chief by Nov. 22.

"This affords Pakistan a last change to immediately address the issues," Maltese Foreign Minister Michael Frendo said.

The Pakistani High Commission in London said late Monday that the country's decisions would not be taken according to deadlines "imposed from the outside."