An office of Arab satellite broadcaster Al-Jazeera in the Egyptian capital was firebombed on Wednesday, a security official said, as protesters clashed near the downtown Tahrir Square.
"Unknown assailants" lobbed Molotov cocktails at the office which overlooks Tahrir, causing a big fire, the official said.
The arson attack gutted the office, with a window overlooking the iconic square smashed in and cameras and lighting charred.
Security officials said that no one had been injured, but the Qatari-owned television network said later in a statement that one of its staff members suffered injuries.
The attack, the channel said, had come after a campaign of "incitment targeting Al-Jazeera network in Egypt over the past days".
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The statement said that the campaign had been "surprising, mainly as Al-Jazeera in Egypt and abroad has never taken sides in any political conflict".
It said there were calls on Egyptian television channels to shut down the offices of Al-Jazeera.
The attack came after the arrest of 118 people since clashes broke out on Monday during a protest to mark the first anniversary of deadly street battles that piled pressure on Egypt's then ruling military to quit power.
Medics said that at least 60 people had been injured in the three days of confrontations. One protester, a member of the April 6 movement, was in critical condition, the group said on Tuesday.
On Monday, clashes erupted as protesters tried to destroy a concrete barrier which security forces put up to end last year's fighting with activists.
At least 45 protesters died in the five days of street battles that began on November 19 last year to put pressure on the military, which took power after a popular uprising overthrew president Hosni Mubarak in February 2011.
The military, which responded by vowing to hand over power by June 2012, transferred control to Islamist President Mohamed Morsi after his election that month.