Muslim mobs set two churches on fire overnight in Cairo during sectarian clashes that left 12 dead and more than 200 injured. The deepening religious violence in military-ruled Egypt added news tensions to an already chaotic and lawless transition to democracy.

Military authorities arrested 190 people, immediately sending them to military prosecutions and threatening the maximum penalty against anyone attacking houses of worship. It was the military's toughest response yet to a series of violent clashes between the two religious groups and signifies swift justice.

Mobs of ultra-conservative Muslims attacked the St. Menas church in the Cairo slum of Imbaba late Saturday following rumours that a Christian woman married to a Muslim man had been abducted. Local residents said a separate mob of youths armed with knives and machetes attacked the Virgin Mary church several blocks away with firebombs.

"People were scared to come near them," said local resident Adel Mohammed, 29, who lives near the Virgin Mary Church. "They looked scary. They threw their firebombs at the church and set parts of it ablaze."

During Egypt's 18-day uprising that ousted former President Hosni Mubarak several months ago, there was a rare spirit of brotherhood between Muslims and Christians. Each group protected the other during prayer sessions in Cairo's Tahrir Square, the epicenter of the revolution.

But in the months that followed the toppling of Mubarak on Feb. 11, there has been a sharp rise in sectarian tensions, fuelled in part by newly active ultra-conservative Muslim movement, known as the Salafis.

The once-quiescent Salafis have become more assertive post-revolution in trying to spread their ultra-conservative version of an Islamic way of life. In particular, they have focused their wrath on Egypt's Christians, who make up 10 percent of the country's 80 million people.

On Friday, a few hundred Salafis marched through Cairo celebrating al-Qaida leader Osama bin Laden and condemning the U.S. operation that killed him.