Millions of protesters staged demonstrations across Egypt Sunday, calling for the resignation of President Mohammed Morsi. As the BBC reports, tens of thousands took to Cairo's central Tahrir Square last night, marking the largest protests since the 2011 revolution that overthrew former President Hosni Mubarak. Opposition groups have given Morsi until Tuesday to resign, but the president has remained defiant, urging Egyptians to follow constitutional order.

Unrest began spreading across the country last week, but reached a crescendo in Cairo last night, as thousands gathered in Tahrir and marched on the presidential palace. In perhaps the most visually surreal moment of the night, hundreds of protesters pointed laser pointers at an army helicopter flying overhead, wrapping the aircraft in a web of green beams. AFP photographer Khaled Desouki captured the moment from afar.

Coordinated to fall on the one-year anniversary of Morsi's inauguration, the demonstrations have been fueled by secular opposition groups that accuse Morsi and his Islamist allies of monopolizing political power. Activists have also expressed disenchantment with Egypt's economic progress under Morsi, with many demanding a new round of presidential elections.

Reports indicate that the protests have been largely peaceful and often festive in nature, though there have been outbursts of violence. On Monday, protestors laid siege to the Cairo headquarters of the Muslim Brotherhood — an Islamist political party aligned with Morsi — before setting the building on fire. According to Egypt's health ministry, 16 people have died since unrest began spreading across the country last week.

Morsi has remained defiant throughout the unrest, urging protesters to obey constitutional order, and dismissing calls for new elections. He also suggested that much of the upheaval has been driven by supporters of Mubarak, who served as president for 30 years before being ousted during the Arab Spring protests two years ago.

It remains unclear whether Egypt's army will intervene as it did in 2011, when it ruled the country during the transition from Mubarak to Morsi. The military has thus far remained neutral, though it warned last week that if Egypt descends into chaos, it will defend the "will of the people." On Monday, Egypt's military chief gave politicians 48 hours to resolve the crisis, urging officials to "meet the demands of the people."

"There is no room for any talk against this constitutional legitimacy," Morsi said in a Sunday interview with The Guardian. "There can be demonstrations and people expressing their opinions. But what's critical in all this is the adoption and application of the constitution. This is the critical point."

Tamarod, the opposition group that organized this week's demonstrations, has vowed to wage a campaign of "complete civil disobedience" if Morsi does not step down by 5 PM local time on Tuesday (10 AM ET).

"There is no way to accept any half measures," Tamarod said in a statement. "There is no alternative other than the peaceful end of power of the Muslim Brotherhood and its representative, Mohammed Morsi."