Dramatic images from Egypt have shown hordes of protesters trying to bring down an army helicopter, by using lasers.
The sky above Cairo was lit up as protesters also used the laser pens to write 'game over' on government buildings in a message to the country's embattled president Mohammed Morsi.
Morsi, who a year ago was inaugurated as Egypt's first freely elected president, was expected to resign or be forced from office after a military deadline for him to solve the crisis passed on Wednesday afternoon.
As the 3pm (UK time) deadline approached, Egyptian soldiers entered state TV buildings in Cairo.
The president had been told to "meet the demands of the people" or the army would intervene.
Thousands of people gathered in Tahrir Square, where the Egyptian revolution first started, Sky News reported, while Muslim Brotherhood supporters said they had been attacked with automatic gunfire.
At least 16 people, mostly supporters of the president, were said to have been killed and about 200 wounded when gunmen opened fire on pro-Morsi demonstrators at Cairo University campus.
As the deadline passed, Morsi was said to be meeting military figures.
Earlier, David Cameron issued a plea for an end to the violence.
He told MPs: "These are deeply disturbing scenes, the level of violence is appalling.
"We should appeal to all sides to calm and stop the levels of violence, and particularly sexual assaults."
He added: "It is not for this country to support any single group or party. What we should support is proper democratic processes and proper government by consent.
"Very clear messages have been sent to president Morsi - including by president Obama who spoke to him directly, and we have also been communicating through our ambassadors - that, yes, he has a democratic mandate and we respect that, but democracy also means ensuring that everyone has a voice and that leaders have a responsibility to represent all Egyptians and show they are responsive to their concerns.
"That's what the government needs to do in order to bring about peace and stability in that country."
The Foreign and Commonwealth Office (FCO) is advising against travel to all regions of Egypt except resorts on the Red Sea in South Sinai and in the Red Sea Governorate on the Egyptian mainland.