The US embassy in Pakistan's capital, Islamabad, has become the latest target of protesters angry at an anti-Islam video that triggered protests in the Arab and Muslim world.
The total number of protesters reached around 5,000 later on Thursday with the arrival of protesters carrying the flags of hardline Islamist groups. At least 50 people were injured as police fired tear gas and live rounds towards the crowds.
Hundreds of students from various colleges and educational institutions in Islamabad had begun clashing with police as security forces tried to block them from reaching the embassy compoud, which also includes the British and French diplomatic missions.
The students responded by pelting the police with stones, and the police retaliated by firing tear gas shells.
Al Jazeera's Kamal Hyder, reporting from Islamabad, said the situation was eventually brought under "control" by police and the military.
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"Police tried to control the crowd by firing in the air and using tear gas. But because of the size of the crowd, the police were forced to ask for help from the military," he said.
Many thousands were trying to march but were prevented by the security forces, said our correspondent. He said military helicopters have been hovering over the area where the protests are taking place.
Several students were injured when policemen hurled the stones back at the crowd. According to local television channels, several policemen were also injured.
"Our policemen are not any better than the Americans because they are trying to stop us. They are in the same league as them, they are heretics like them," said student Jawad Ahmed.
"They should allow us to demolish the American embassy because they have blasphemed against our holy Prophet. The police are also becoming an accomplice of blasphemers."
Protests took place in other parts of Pakistan as well. In Lahore, the cultural capital of the country, students from an Islamic school marched through the streets and chanted anti-American slogans.
In Muzaffarabad, the capital of Pakistani-administered Kashmir, more than 200 protesters set fire to an effigy of Barack Obama, the US president.
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"We will make known it by burning the effigy of Obama and his allies that a blasphemer deserves to be burnt to death," said Maulana Mehmood ul Hassan Ashraf, secretary general of Jamiat-e-Ulema-e-Islam.
Large protests are expected on Friday, the weekly Islamic day of prayer, which has been declared a special public holiday in Pakistan, said our correspondent.
Hyder said the government was nervous and that several containers had already been brought in, as roads entering Islamabad were being blocked ahead of tomorrow's protests.
'Represented the views'
The US has bought time on Pakistani television stations to run a series of ads in a bid to stem Muslim protests, a US official said on Thursday.
The State Department has spent some $70,000 to air the ad in Urdu across seven Pak
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istani TV channels in a bid to dissociate the US government from the inflammatory film, spokeswoman Victoria Nuland said.
"After the video came out there was concern in lots of bodies politic, including in Pakistan, whether this represented the views of the US government," Nuland told journalists.
"So in order to ensure we reached the largest number of Pakistanis, some 90 million as I understand in this case with these spots, it was the judgment that this was the best way to do it."
The aim was "to make sure that the Pakistani people hear the president's messages and the secretary's messages", Nuland said of the ad, which was produced and edited by the US embassy in Islamabad.
Obama is shown saying: "Since our founding the United States has been a nation that respects all faiths. We reject all efforts to denigrate the religious beliefs of others."
Clinton then says: "Let me state very clearly, the United States has absolutely nothing to do with this video. We absolutely reject its contents. America's commitment to religious tolerance goes back to the very beginning of our nation."
State department warning
The protests coincided with a warning to US citizens from the state department to avoid travelling to Pakistan.
Officials upgraded their ongoing advisory about travel risks in Pakistan, explicitly advising Americans to put off any non-essential travel to the country. They "strongly urged" those who are already there to avoid protests and large gatherings.
The state department said the presence of al-Qaeda, Taliban elements and "indigenous militant sectarian groups poses a potential danger to US citizens throughout Pakistan".
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Protests broke out across the Arab and Muslim world, as well as some Western countries, when the video denigrating the Prophet Muhammad, Islam's most revered figure, was posted on YouTube. Many Muslims regard any depiction of the prophet as blasphemous.
In Benghazi, Libya, a US consulate was attacked by angry rioters on September 11, killing the US ambassador to Libya and three of his colleagues.
Obama said on Wednesday the man behind the anti-Islam movie was a "shadowy character".
The White House said the matter was still under investigation and that the situation in Libya was not hopeless although the road to a more stable future was "bumpy and at times dangerous".