By Zeeshan Haider

PESHAWAR, Pakistan, Dec 18 (Reuters) - Pakistani Islamists called on Thursday for the government to block vital supply routes for Western forces in Afghanistan in response to U.S. missile strikes on al Qaeda and Taliban targets in Pakistan.

Militants have launched a string of attacks in recent months aimed at choking off supplies trucked through the Khyber Pass, but while some have been disrupted, most are getting through.

Chanting anti-American slogans, about 5,000 activists of the Jamaat-e-Islami political party, Pakistan's best-organised Islamic party, paraded through the northwestern city of Peshawar on Thursday.

"We consider the presence of American forces in Afghanistan a big conspiracy against Pakistan," party leader Qazi Hussain Ahmed told the protesters.

"They take their supplies to Afghanistan on our roads and in return they kill our people with bombs. This must be stopped," he said to chants of "Down with America" and Allahu Akbar (God is Greatest).

The government of President Asif Ali Zardari is likely to ignore the call to block supplies, but deteriorating security in the northwest is a major problem and the Islamists' protest could prove to be a focus for dissatisfaction against a backdrop of rising unemployment and prices.

The U.S.-led campaign against militancy is deeply unpopular across Pakistan, especially in the ethnic Pashtun northwest.

The U.S. military sends 75 percent of supplies for the Afghan war through or over Pakistan, including 40 percent of the fuel for its troops, the U.S. Defense Department says.

INTENSIFIED ATTACKS

At the same time, U.S. forces, apparently frustrated by Pakistan's inability to tackle militants in border enclaves and alarmed by deteriorating Afghan security, have intensified missile attacks on militants in Pakistan by pilotless drones.

The attacks have infuriated Pakistan which, although it is a U.S. ally, says the strikes violate its sovereignty and are counter-productive in its efforts to fight militants.

Pakistani authorities briefly blocked the Khyber route in September after U.S. forces launched a helicopter-borne ground assault on a Pakistani border village.

Authorities said the route was blocked for security reasons, but it illustrated how vulnerable U.S. supplies were.

The Thursday protest was the first major rally by Islamists in Peshawar for several years.

Ahmed did not refer to the militant attacks on the supply routes and urged the crowd to show opposition peacefully.

"Don't take up guns in your hands but greet these convoys with black flags as they pass by your homes to force them to stop," he said.

The Jamaat-e-Islami and other hardline Islamist groups rose to power in North West Frontier Province by exploiting anti-U.S. sentiment in 2002 elections. They were routed in February polls by secular and Pashtun nationalist parties.

About 300 trucks delivering supplies for the Western forces in Afghanistan have been destroyed in a string of attacks by militants near Peshawar over the past two weeks.

At least three truck drivers have been killed and many truckers have stopped taking supplies on the northern route.

Nasir Khan, a customs official on the border, said supply traffic was down by about half: "About 500 to 600 trucks used to cross daily but now only 200 to 300 truck cross."

About 150 trucks crossed on Thursday. (Editing by Robert Birsel and David Fox)

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