In her strongest challenge yet to President Pervez Musharraf's [Images] rule, former premier Benazir Bhutto [Images] on Tuesday asked him to quit and vowed never to serve under the "dictator" after she was placed under house arrest to foil her plans to lead an anti-emergency rally, which kicked off in Lahore [Images] without her.

Fifty-four-year-old Bhutto also said that it was unlikely that her Pakistan People's Party would participate in the general election in January and indicated willingness to form an alliance with other opposition leaders, including exiled former premier Nawaz Sharif.

"We say Musharraf must leave. The time for dictatorship is over. It's time to bring a transfer to democracy," said Bhutto, who ruled out serving under Musharraf in any future governments.

"It seems unlikely that the People's Party will participate in the upcoming election," she told reporters over phone from the house in Lahore, where she is detained.

The election "seems like nothing more than a stage-managed show to return (ruling) Pakistan Muslim League-Q to power... Now we've come to the conclusion that even if we get power, it will just be a show of power, not substantive power," she said.

Bhutto's remarks came after the government launched a renewed crackdown on the opposition by placing her under house arrest for the second time in five days. Earlier on Friday she was put under house arrest in Islamabad to prevent her from addressing a rally in Rawalpindi.

However, her party started its "long march" against the emergency rule with a convoy of over 100 vehicles moving towards the capital Islamabad without Bhutto, who had been served a seven-day detention order.

Shah Mahmoud Qureshi, the president of the Punjab unit of the PPP, led the march with 110 vehicles and thousands of followers, party spokesperson Sherry Rehman said.

About 600 policemen stood guard since midnight outside the home of PPP leader Latif Khosa, where Bhutto is staying in Lahore, preventing her and other party leaders from stepping out to lead the "long march" to Islamabad.

Earlier Bhutto told BBC that "there are no circumstances in which I could see myself serving with General Musharraf... I think Musharraf should leave. He's lost the confidence of people."

This was for the first time that Bhutto sought the resignation of Musharraf as president. Previously, she had been demanding an end to emergency rule and that the general step down as Army chief.

Accusing the military ruler of giving up the "democracy roadmap," she said that polls under emergency will be a sham.

Bhutto said, "I have tried for more than a year... I worked out a roadmap to democracy. But he went back on that and imposed 'martial law'."

The PPP chairperson also said she was in touch with several opposition leaders, including Sharif, Jamaat-e-Islami chief Qazi Hussain Ahmad and Awami National Party leader Asfandyar Wali Khan, to form a "coalition of interests" to work for democracy.

"The PPP is the country's largest party and we want to have such a coalition," Bhutto told Geo TV.

"We want a coalition with a common minimum platform to put pressure on Musharraf to step down. A caretaker government of national consensus should be formed to supervise elections."

A seven-day detention order was shown to Bhutto's aide Naheed Khan and pasted on the door of the house where she was staying in Lahore, top city police official Aftab Cheema said.

Police cordoned off the area around the house situated in a posh locality in Lahore and set up several metal and barb wire barricades to prevent PPP workers and the media from approaching it.

Trucks filled with sand and two armoured personnel carriers were used to block nearby streets.

As PPP activists and leaders began gathering in the area, dozens of them, including members of the provincial assembly, were bundled into police vans and taken away.

Women PPP workers jostled with police and tried to remove the barb wire but were pushed back.

The PPP's march was aimed at pressurising Musharraf to end the emergency and give up the post of army chief by November 15.

Sherry Rehman, who was in the house with party leaders Khosa, Makhdoom Amin Fahim and Rehman Malik, said Bhutto was being illegally detained and the house had been declared a "sub-jail".

"The PPP's intentions are clear -- we are against the emergency. Party activists are being mobilised across Punjab. Despite the arrests we are determined to fight for democracy," Rehman said.

Bhutto, who returned to Pakistan on October 18 after eight years in self-imposed exile, claimed that 7,000 of her supporters were arrested overnight.

The home department of Punjab province, which issued the detention order, said there were intelligence reports that suicide bombers had entered Lahore to target Bhutto, who survived a terror attack on her homecoming procession, which left nearly 140 people dead.

The government had cited similar security threats to detain Bhutto in her home in Islamabad on November 9.

In Karachi and Peshawar, PPP activists protested Bhutto's detention, clashing with police and burning tyres to block roads.

After the "long march" began, police tried to stop the vehicles at several places and arrested some leaders, but the motorcade was moving towards the capital, Qureshi said on phone.

There was tension in the Lyari area of Karachi, a PPP stronghold, as party workers forcibly shut shops to protest Bhutto's detention.

A group of PPP activists also protested outside the Press Club in Karachi and PPP supporters fired shots at two police stations, though no one was hurt, media reports said.

However, PPP denied that any of its activists was responsible for the shooting incident.

Police used teargas and batons to disperse hundreds of protesters in both Karachi and Peshawar.

Hundreds of PPP workers were also arrested in overnight swoops by police across Punjab province, especially at places where Bhutto had planned to address rallies during the nearly 300-km march that could last for three days.

Interior Ministry spokesman Javed Iqbal Cheema said that Bhutto was placed under house arrest following "credible intelligence reports" that terrorist groups had planned to target her.

He also dismissed reports that Bhutto might be flown from Lahore to Karachi or the capital Islamabad in a military aircraft.