KABUL - Thousands of protesters took to the streets of the Afghan capital on Monday, setting fire to cars and shouting "death to America", the latest in demonstrations that have swept the Muslim world against a film mocking the Prophet Mohammad.Western embassies across the Muslim world are on high alert and the United States has urged vigilance after days of anti-American violence provoked by the film."Agenda for revenge"The violence is the most serious wave of anti-American protests in the Muslim world since the start of the Arab Spring revolts last year. At least nine people were killed in protests in several countries on Friday.The crisis presents US President Barack Obama with a foreign policy headache as November elections approach.Some US officials have suggested the Benghazi attack was planned by Islamist militants using the video as a pretext, a hypothesis endorsed by Mohammed Magarief, the president of Libya's national assembly."Call it whatever you want, al-Qaida or not, what happened was an act by a group with an agenda for revenge. They chose a specific time, technique and certain victims. This is what it was all about," Magarief told Reuters in an interview.However, the US ambassador to the United Nations, Susan Rice, said on Sunday talk shows that preliminary information indicated that the attack was not pre-meditated."There's no question, as we've seen in the past with things like 'The Satanic Verses', with the cartoon of the Prophet Mohammad, there have been such things that have sparked outrage and anger and this has been the proximate cause of what we've seen," she said.US forces deployedUS Defense Secretary Leon Panetta said at the weekend he hoped the worst of the violence was over but US missions must remain on guard."It would appear that there is some levelling off on the violence that we thought might take place," he told reporters on his plane en route to Asia on Saturday."Having said that, these demonstrations are likely to continue over the next few days, if not longer."The United States has deployed a significant force in the Middle East to deal with any contingencies and rapid deployment teams were ready to respond to incidents, he said.The foreign minister of Egypt, where hundreds of people were arrested in four days of clashes, assured Secretary of State Hillary Clinton that US diplomatic grounds would be protected.Mohamed Kamel Amr told Clinton in a telephone call that the film was designed to incite racial hatred and was therefore "contradictory with laws aimed at developing relationships of peace and mutual understanding between nations and states"."There were between 3,000 and 4,000 demonstrators. They burned some police cars, but we could split them up and prevent the insecurity widening," Lieutenant-General Fahem Qayem, police quick reaction force commander, told Reuters in Kabul.Embassies in Kabul's heavily guarded central zone were placed on lockdown, including the US and British missions, after violence flared near fortified housing compounds for foreign workers in the city's volatile eastern suburbs."We will defend our prophet until we have blood across our bodies. We will not let anyone insult him," said protester Jan Agha Pashtun, giving what was apparently a false name to avoid police retaliation. "Americans will pay for their dishonor."It was the latest in a week of violent protests fanned by anger over a video, posted on the Internet under several titles including "Innocence of Muslims", that mocked the Prophet Mohammad and portrayed him as a womanizer and a fool.The head of Libya's national assembly said an attack on the US consulate in Benghazi that killed Ambassador Christopher Stevens and three other Americans last Tuesday looked like a planned assault by a "group with an agenda" rather than a spontaneous reaction to the video posted online.With protests against the film continuing from London to Lahore on Sunday, Western diplomatic missions were on edge. Germany followed the US lead and withdrew some staff from its embassy in Sudan, which was stormed on Friday.The United States ordered non-essential staff and family members to leave its embassy on Saturday after the Khartoum government turned down a US request to send Marines to bolster security.Non-essential US personnel have also been withdrawn from Tunisia, and Washington urged US citizens to leave the capital Tunis after the embassy there was targeted on Friday.About 350 people chanted slogans at a rally outside the US embassy in London on Sunday. A small group of protesters burned a US flag outside the embassy in the Turkish capital, and in Pakistan there were protests in more than a dozen cities.The head of Shi'ite militant group Hezbollah called for protests in Beirut this week."Those responsible for the film, starting with the US, must be held accountable," Sayyed Hassan Nasrallah said. "All these developments are being orchestrated by US intelligence."