Hundreds of Afghans demonstrated Monday in support of President Hamid Karzai's threat to send troops against Taliban militant leaders in Pakistan, with many protesters saying they were ready to take up arms.

Across the border, Pakistan summoned the Afghan ambassador to lodge a protest, saying it too would defend itself.

Afghan tribesmen, elders and clerics gathered in at least four cities in eastern Afghanistan — where Karzai's support is strongest.

"We are ready to defend our homeland," said Malik Majid, an elder in Paktika province, near the Afghan-Pakistan border. "We strongly condemn Pakistan's interference in our country."

President Bush, speaking in London, said the United States can help calm the "testy situation," but refused to endorse Karzai's threat. He called on leaders of Afghanistan and Pakistan to hold talks and share intelligence.

"It's in no one's interest that extremists have a safe haven from which operate," he said.

On Sunday, Karzai threatened to send troops to target Taliban leaders Mullah Omar and Baitullah Mehsud in Pakistan.

Omar led the Taliban regime that was ousted from power in Afghanistan in the U.S.-led invasion in 2001. Kabul claims Pakistan shelters Omar, but Islamabad maintains the elusive militant chief is in Afghanistan.

Mehsud is the most prominent leader of Pakistani Taliban militants and was accused in the killing of former Pakistani prime minister Benazir Bhutto, a charge he denied. Mehsud recently told reporters that he is sending fighters to battle U.S. troops in Afghanistan.

In response to Karzai's statement, Pakistan summoned the Afghan ambassador.

Foreign Minister Shah Mahmood Qureshi described Karzai's statement as "threatening" and "regrettable." Qureshi said he would make "it absolutely clear that Pakistan shall defend its territorial sovereignty."

Karzai's threat — the first time he has said he would send forces into Pakistan — comes only days after a sophisticated Taliban assault on Kandahar's prison freed 870 prisoners, including hundreds of Taliban militants.

Although Afghanistan appeared unlikely to follow through on the threat, the spat marked a new low in relations between two key nations in the U.S.-led war on terror, often strained over allegations of cross-border militancy.

Karzai has long pleaded with Pakistan and the international community to confront Taliban safe havens in tribal areas, and U.S. officials have increased their warnings in recent weeks that such sanctuaries must be dealt with.

Afghan officials reported demonstrations at several locations on Monday.

In Sharon district of Paktika, governor spokesman Ghami Mohammad Yar said hundreds of tribesmen, elders and clerics had gathered at the governor's compound to express support for Karzai.

"We are ready to sacrifice, like before, for the protection of our homeland borders," Yar said, referring to the resistance against Soviet invaders in the 1980s.

Taliban militants themselves often compare their own fight against U.S.-led forces in Afghanistan to the anti-Soviet jihad.

Karzai, like much of the population in eastern Afghanistan, is ethnic Pashtun. Many are tired of militancy in a country scarred by a quarter century of war and support democracy in Afghanistan.

But support for Karzai's U.S.-backed administration is also running low. It appears there is no shortage of Taliban recruits among Pashtun communities on both sides of the Afghan-Pakistan border.

Pakistan says the root causes of the insurgency are the failure of Karzai's government and international community to address the grievances of Pashtuns and develop the region.

Support of the U.S.-led war on terror is unpopular in Pakistan — a sentiment that intensified after U.S. aircraft dropped bombs along the frontier last week during a clash with militants. Pakistan said the incident killed 11 of its paramilitary forces.

In a sign of that sentiment, Sadiqul Farooq, a spokesman for the second-largest party in Pakistan's fractious ruling coalition, advocated the withdrawal of foreign forces from Afghanistan as "an effective solution of the Afghan problem."

A spokesman for Pakistan's Taliban movement has warned of an escalation in Taliban attacks against NATO and Afghan forces if Karzai sends forces across the border. Maulvi Umar also said the Afghan army would face defeat at the hands of thousands of tribal fighters.