KARACHI, Pakistan - Authorities marshaled 15,000 security forces in Pakistan's largest city yesterday, fearing clashes during rival protests today by opponents of President Pervez Musharraf and a pro-government party with a history of violence.

Government opponents hope to hold their biggest demonstration yet against Musharraf's decision to suspend the head of the Supreme Court two months ago, a move that has plunged Pakistan into deepening political turmoil.

The Mutahida Qami Movement, a pro-government party with a support base in Karachi, has announced it will hold a counterdemonstration less than a mile away. In the 1990s, scores of party members were arrested for allegedly kidnapping dozens of their rivals and attacking security forces. Party members are still heavily armed, but critics say they enjoy impunity as part of Musharraf's government.

"If something happens to the chief justice, or any of his supporters in Karachi or elsewhere, the government alone would be responsible for it," said Farhatullah Babar, spokesman for the opposition Pakistan Peoples Party.

Opponents accuse Musharraf of trying to sideline the independent-minded Chief Justice Iftikhar Mohammed Chaudhry in case of legal challenges to his plan to seek a new five-year term in elections this fall.

Musharraf, a key U.S. ally who took power in a bloodless 1999 coup, denies that Chaudhry's suspension was politically motivated. He said there was serious evidence that the judge abused his position.

Opposition groups have joined thousands of lawyers in demonstrations around the country over the last month to demand Chaudhry's reinstatement, sometimes skirmishing with police. Chaudhry, who is expected to address today's demonstration, drew nearly 20,000 supporters at a protest last weekend in the eastern city of Lahore.