SIRNAK, Turkey — Dozens of Turkish military vehicles loaded with soldiers and heavy weapons rumbled toward the Iraq border on Monday after an ambush by rebel Kurds that killed 12 soldiers and left eight missing.

The guerrilla ambush on Sunday outraged an already frustrated public. Demonstrations erupted across the country and opposition leaders called for an immediate strike against rebel bases in Iraq, despite appeals for restraint from Iraq, the U.S. and European leaders.

Prime Minister Recep Tayyip Erdogan said he told Secretary of State Condoleezza Rice in a telephone conversation on Sunday night that Turkey expected "speedy steps from the U.S." in cracking down on Kurdish rebels and that Rice asked "for a few days" from him.

Erdogan did not specify what he meant by "speedy steps," but he has often urged the United States and Iraq to crack down on the Kurdistan Workers' Party, or PKK. Turkish leaders say it is the responsibility of those countries to do whatever is necessary to destroy the guerrilla group's bases in northern Iraq.

"Beyond emphasizing our righteousness, she conveyed how seriously they take the matter by saying 'Please give us a few days,"' Erdogan said.

The Turkish military confirmed Monday that eight of its soldiers were missing after the ambush by Kurdish rebels that left 12 other soldiers dead and brought the northern Iraq border area to the brink of war. The military said its counteroffensive left 34 rebels dead.

"Despite all search efforts, no contact has been established with eight missing personnel since shortly after the armed attack on the military unit," the military said in a statement on its Web site.

The pro-Kurdish Firat news agency, based in Belgium, released the names of seven people it said were Turkish soldiers captured by separatist fighters in Sunday's ambush. It said an eighth soldier was also taken captive but did not release his name.

An AP Television News cameraman saw a convoy of 50 military vehicles, loaded with soldiers and weapons, heading from the southeastern town of Sirnak toward Uludere, closer to the border with Iraq.

It was unclear whether the vehicles were being sent to reinforce troops engaged in fighting with rebels on Turkish soil or were preparing for possible cross-border action. Tens of thousands of Turkish troops are already deployed in the border area.

More than 2,000 protesters in Istanbul, mostly members of an opposition party, denounced the attack by the PKK and urged the government of Erdogan to resign, the private Dogan news agency reported.

In Ankara, hundreds convened at a main square shouting "Down with the PKK and USA!" "We'll go into Iraq and we'll hang Barzani," and "Apo's dogs can't bring us down!". Massoud Barzani is the leader of Iraq's Kurdish region where PKK rebels have bases; Apo is Kurdish rebel leader Abdullah Ocalan's nickname.

Ambulances decorated with Turkish flags drove around main streets, their sirens on.

Some 13,000 schoolchildren in Bilecik in eastern Turkey held a minute of silence while people marched down a main street, waving the Turkish flag, the state-run Anatolia news agency reported.

In Bursa, in northwest Turkey, some protesters walked to a military conscription office and asked to enlist to fight rebels.

Turkey's military said Sunday it had launched an offensive backed by helicopter gunships in retaliation for the attack, shelling rebel positions along the rugged Turkish-Iraqi border.

The rebel attack occurred four days after Parliament authorized the government to deploy troops across the border in Iraq, amid growing anger in Turkey at perceived U.S. and Iraqi failure to live up to pledges to crack down on the PKK.

The United States opposes any unilateral action by Turkey, fearing it could destabilize the most stable part of Iraq.

Sunday's attack raised the death toll of soldiers in PKK attacks in the past two weeks to around 30.

Rebels periodically cross the border to stage attacks in their war for autonomy for Turkey's predominantly Kurdish southeast. More than 30,000 people have died in the conflict that began in 1984.

Out of respect for the soldiers killed Sunday, a concert by American R&B singer Beyonce Knowles in Istanbul was canceled, a soccer club that was organizing the event said on its Web site Monday.

Beyonce was to give a concert in Istanbul on Wednesday, as part of her world tour.

"It was not possible for (the concert) to be held in such an atmosphere," the Web site quoted Ali Koc, vice president of the soccer club Fenerbahce.