* Studio used by Al Jazeera set on fire in Tahrir Square
* Street violence enters third day, dozens wounded
CAIRO Nov 21 (Reuters) - A studio used by the Al Jazeera TV network was set on fire on Wednesday in central Cairo as security forces and protesters fought in the streets of the Egyptian capital for a third day.
Police said they had arrested three men on suspicion of being involved in the attack and gave no details on their identity or any affiliations.
The violence that has wounded dozens and led to more than 100 arrests grew out of protests called this week to mark the first anniversary of deadly street battles between police and protesters opposed to army rule.
Egypt's streets have been calmer since President Mohamed Mursi took power from the generals in July, after winning a democratic presidential vote. Demonstrations by political and labour activists remain frequent however, as Egyptians exercise their new right to protest.
State TV referred to the violence in central Cairo as "organised sabotage". In a statement, the government condemned the violence and listed restoring security as a priority.
"Everybody is a citizen of this nation, be they security men, soldiers or others. All should be aware that the stone that is thrown wounds everyone, and the Molotov (petrol bomb) that is lit burns the nation," a cabinet statement said.
The studio operated by Al Jazeera Mubasher Misr, a station set up to cover domestic affairs after the uprising that toppled President Hosni Mubarak, was badly damaged by the blaze.
Overlooking Tahrir Square, its windows were smashed and two empty bottles, apparently used for petrol bombs, were found inside the office, Reuters TV footage showed.
"There were 200 to 250 people gathered outside the studio chanting against the channel," Ahmed Dessouki, a producer with the channel, told Reuters television.
The Interior Ministry described the perpetrators as "trouble makers" who had attacked police officers when they had arrived to investigate, the state news agency reported. The public prosecutor has ordered an investigation.
The studio is around the corner from streets that have been the main site of this week's clashes and where several hundred youths congregated for a third consecutive day on Wednesday, some of them throwing rocks and lighting fires.
Television footage showed youths throwing objects at buildings, including the American University in Cairo. At one point, a group tried to break down the university gate.
Officials said 118 people had been arrested and 76 wounded since Monday evening, when the trouble began.
Last year's street battles started when police pulled down the tents of protesters who had camped overnight in Tahrir Square after a demonstration against the generals.
That prompted thousands of protesters to return to the square, triggering clashes, later known as "Mohamed Mahmoud events" after the street in which they took place.