Police detained protest leaders Thursday and stopped hundreds of marchers from leaving Pakistan's largest city for a rally in the capital, underscoring the government's determination to squelch the demonstration.

Pakistan's one-year old government is facing growing opposition from lawyers and political foes demanding it fulfill a promise to reinstate judges fired by former President Pervez Musharraf. The showdown threatens to destabilize the country at time of rising militant violence, a punishing economic crisis and increasing dissatisfaction with President Asif Ail Zardari.

Authorities had allowed a group of several hundred protesters to set off from downtown Karachi earlier, but stopped them as their convoy of cars and buses tried to enter a highway leaving the city. An Associated Press reporter saw police bundle several local political party leaders into vans, while other protesters sped back into the city.

Several people sat on the road chanting "Zardari is a traitor! Zardari a dog!" before police arrested them, triggering brief scuffles.

Police officer Suleman Syed said officers were instructed to let no protesters onto the road.

The protest movement picked up steam when the Supreme Court banned opposition leader Nawaz Sharif, who is a former prime minister, and his brother from elected office. After the ruling, the federal government dismissed the Punjab provincial administration led by Sharif's brother, stoking popular anger.

The lawyers, Sharif's party and other small groupings planned to converge on the parliament building in Islamabad on Monday from cities across the country and remain there until their demands are met. The government has banned protests in much of the country and detained more than 360 activists, but the lawyers say they are determined to press on.

Another group of several hundred protesters left from the southwestern city of Quetta. The protests are expected to gather strength as people in Punjab — Sharif's political powerbase — set off for the capital over the weekend.

The arrests reminded many Pakistanis of similar moves against many of the same activists by Musharraf in 2007 — clampdowns that dramatically reduced his popularity, ushered in the new government, and contributed to his ouster the following year.

U.S. Ambassador Anne Patterson met with Sharif Thursday in a bid "to get things resolved" between him and the government, Sharif spokesman Sadiqul Farooq said.

"It is not only the American ambassador, other friendly countries are in contact," he said.

The U.S. Embassy said it does not comment on the ambassador's meetings, but foreign help in resolving political disputes in Pakistan has been common in the past.

In recent days, leaders of other political parties have scuttled between Sharif, Zardari and Prime Minister Yousuf Raza Gilani, seeking a solution to the crisis, but there has been little sign that one is forthcoming.

The growing political unrest is raising the specter of a possible military intervention in a nuclear-armed nation prone to army coups. It could also put Washington in a prickly position if the civilian government — which itself rose to power on the back of the rallies and marches against Musharraf — keeps clamping down on dissidents.

Zardari, the widower of assassinated former Prime Minister Benazir Bhutto, has cultivated ties with the U.S. and sought to rally Pakistanis behind the fight against Islamic extremists. Sharif is considered closer to Islamic parties and conservative factions less inclined to support the U.S. war effort in neighboring Afghanistan.

Protesters have pledged a peaceful march, but Sharif has used words like "revolution" in recent speeches, prompting the government to warn him against committing sedition.

Information Minister Sherry Rehman told reporters the rallies were banned to "avoid bloodshed in the streets." While acknowledging her party had staged similar rallies in the past, she insisted that "we never called to raise the flag of rebellion."

The ruling party has restored most of the judges fired by Musharraf, but a few, including a former Supreme Court chief justice, have not regained their seats.

Zardari is believed to fear that those judges could move to limit his power or reopen corruption cases against him that were dropped by Musharraf when the former general was seeking to forge a political alliance before last year's elections.