Rizwan Tabassum, Getty Images
Pakistani lawyers chant against President Pervez Musharraf during a march in Karachi on Thursday. Pakistan's ruling coalition agreed to impeach Musharraf.

ISLAMABAD, Pakistan — A move by Pakistan's usually fractious governing coalition on Thursday to impeach President Pervez Musharraf left the country on the brink of a political crisis that threatened to paralyze the government at a critical moment when the United States is demanding greater action against militants based here.

The governing coalition set no formal deadline for the start of impeachment proceedings against Musharraf, a favored U.S. ally, leaving open the possibility of a protracted and debilitating political fight that could take months of haggling to secure the parliamentary votes needed for impeachment. The actual charges have yet to be announced.

It also raised the threat that Musharraf would try to dissolve the Parliament, or that he would look to the army for protection, though many analysts said the military was unlikely to intervene.

The announcement that the civilian leaders would seek impeachment was the culmination of months of wrenching political changes after the assassination of opposition leader Benazir Bhutto in December and the decisive victory of her party in elections in February. Since then, the leaders of the country's two major parties, Asif Ali Zardari and Nawaz Sharif, have forged a tense governing coalition that has teetered on collapse.

Zardari, the head of the Pakistan Peoples Party, and Sharif, the leader of the Pakistan Muslim League-N, have barely been on speaking terms. For the past several days, they were closeted in meetings on how to keep their fractious coalition together.

Sharif has pushed Zardari to join impeachment proceedings against the president. Zardari had resisted, but this week he apparently decided that the one way to keep the coalition functioning was to undertake a frontal attack on Musharraf, who is immensely unpopular.

On Thursday, the two coalition leaders issued a joint communique saying their government would "immediately initiate impeachment proceedings" and that it would "present a charge sheet against General Musharraf."

Musharraf was described by his allies as determined to fight back, and met all day on Thursday with his political backers and his constitutional lawyer, Syed Sharifuddin Pirzada. In an indication of the gravity of his situation, the president called off his trip to attend the opening of the Olympic Games.

Many Pakistani officials said they believed Musharraf would seek support from the Bush administration. It has endowed Pakistan with more than $12 billion of mostly military aid since the Sept. 11 attacks for its cooperation in combating the Taliban and al-Qaida.