Pakistan's Supreme Court ordered the arrest of the country's prime minister and tens of thousands of antigovernment protesters demanded immediate elections, whipping up political chaos in a country the U.S. sees as vital in helping it wind down the war in Afghanistan.

Tuesday's events marked a twin blow to the government of President Asif Ali Zardari, which has been locked in a three-way battle for power with the country's Supreme Court and the nation's powerful military since it came to power in 2008.

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