ISLAMABAD, Pakistan — The five-month-old coalition government in Pakistan collapsed Monday when the head of the minority party, Nawaz Sharif, announced his members would leave the fractious alliance, citing broken promises by Asif Ali Zardari, the leader of the majority party.

“We have been forced to leave the coalition,” Mr. Sharif said in Islamabad. “We joined the coalition with full sincerity for the restoration of democracy. Unfortunately all the promises were not honored.”

The exit by Mr. Sharif’s party, the Pakistan Muslim League-N, had been expected in the last few days, and was finally spurred by the decision of Mr. Zardari to run for president, in an electoral college vote set for Sept. 6. President Pervez Musharraf resigned last week under threat of impeachment.