ISLAMABAD (Reuters) -Pakistan’s Islamist opposition on Monday vowed more protests against a U.S. airstrike aimed at Osama bin Laden’s deputy that killed at least 18 civilians, and commentators said the raid would boost support for militants.

Intelligence officials said U.S. counterparts had told them the order for Friday’s strike was based on information al Qaeda’s second-in-command, Ayman al-Zawahri, had been invited to a dinner at a village in the Bajaur tribal region near the Afghan border.

Two clerics known for harboring al Qaeda militants attended but left hours before the raid and Zawahri did not show up, they said. U.S. intelligence sources said Zawahri’s fate would not be known until remains of the dead were examined.

Although the U.S. government has not publicly acknowledged any responsibility, intelligence officials in Washington with knowledge of the operation said it was aimed at Zawahri and said they believed drone aircraft armed with missiles were used.

Pakistan, a key U.S. ally in the war on terrorism, condemned the attack and summoned the U.S. ambassador to protest at the deaths of civilians, who included women and children.

The incident provoked anti-American protests in several Pakistani towns and cities on Sunday, and supporters of a party allied with President Pervez Musharraf were among 10,000 people who rallied in the southern city of Karachi.

The Muttahida Majlis-e Amal (MMA) opposition Islamist alliance has called for a debate in parliament on the issue this this week, which could prove uncomfortable for Musharraf.

Musharraf, who is also commander-in-chief of the armed forces, already faces criticism for his handling of separatist unrest in Baluchistan province and over plans for major infrastructure projects.

The MMA said it planned more protests after consultations with other opposition parties.

“We will continue to protest until American forces withdraw from Afghanistan,” said its spokesman, Shahid Shamsi.

CYNICMISM IN PAKISTAN

The incident — which came just days after Pakistan lodged a strong protest with U.S.-led forces in Afghanistan, saying cross-border firing in a nearby tribal area had killed eight people — has been greeted with widespread cynicism in Pakistan.

“America raised the bogey of Zawahri to provide justification for this attack,” Meraj-ul-Huda, an MMA leader, told the Karachi rally, echoing remarks by Pakistani officials speaking privately.

Newspaper editorials denounced the attack and warned it would fuel support for Islamist militants.

“The strike would be unjustified even if it took out the targeted figures,” said The News, an English-language daily.

“Apart from the element of Pakistan’s sovereignty, this country is the United States’ partner in the ‘war on terror’,” it said, adding it could “supply an excellent new case for the fanatics.”

The Daily Times said there was reason to believe it was a joint operation based on information from the Pakistani side.

It only added to Musharraf’s problems, the paper said, pointing to a long-running and bloody struggle against al Qaeda-linked militants in another tribal area and government efforts to suppress the Baluchistan insurgency.

“There is a growing threat from the grand opposition of the country of uniting and marching on Islamabad,” it said.

Even though Sunday’s protests were not large by Pakistani standards, worrying for Musharraf will be the fact they were joined by the Muttahida Qaumi Movement (MQM), an important ally that has criticized him over Baluchistan. MQM leader Altaf Hussain has promised more protests if there are more air strikes.