In a massive protest, hard-line Islamic leaders have warned the Pakistani government against caving to U.S. pressure and releasing an American official arrested in the shooting deaths of two Pakistanis.

The party chiefs of Jamaat-e-Islami and Jamiat Ulema Islam spoke alongside other hard-liners Sunday at a rally of at least 15,000 people in the eastern city of Lahore, where the shootings took place. Many protesters called for the American official to be hanged.

The U.S. has said the American acted in self-defense when he shot two armed men who tried to rob him Thursday. The embassy has said he has diplomatic immunity and has accused Pakistan of illegally detaining him.

The Pakistani government has refused to release him and says the courts should decide his fate. They say the American will not be handed over to the U.S. Embassy until Pakistani authorities first complete their investigation.

On Saturday, the U.S. Embassy said the American was a U.S. diplomat carrying a diplomatic passport and a valid visa, describing him as a member of the technical and administrative staff covered by diplomatic immunity.

The Vienna Conventions state that diplomats must not be liable for any form of arrest or detention and are immune from civil or criminal prosecution.

The U.S. asserts that the arrest of the American, who has been charged with double murder, is a violation of the diplomatic conventions.

Acknowledging the potential of the case to inflame anti-American sentiments, one senior American source said a lot could be jeopardized “if it goes South.”

NPr’s Julie McCarthy contributed to this report, which includes material from The Associated Press Copyright 2011 National Public Radio. To see more, visit http://www.npr.org/.