(Reuters) - One Christian youth was shot dead Tuesday during a protest between 1,300 Christians and Muslims who were throwing stones and Molotov cocktails at each other, a security source said.

The fighting erupted when a group of Christians blocked a main highway south of Cairo and clashed with Muslims who wanted to pass through, a security source said.

Christians were protesting for the second day over a church in Helwan on the outskirts of the capital that was set on fire Sunday after a row sparked by a relationship between a Christian man and a Muslim woman, witnesses and a security source said.

The army fired gunshots in the air to break up the riot but was unable to quell the fight straight away, witnesses said. An army statement said it "successfully handled riots Tuesday."

One 18-year old Christian was killed by a bullet that struck him in the back but it was unclear who had fired the shot or whether it had been aimed intentionally, the security source said. Some witnesses said they saw protesters carrying weapons.

"They (the Christians and the Muslims) began fighting because people in their cars wanted to pass through and the protesters had blocked the highway," the security source said.

About 20 people were injured and five cars were burned during in the latest sectarian flare-up.

In downtown Cairo hundreds of Christian protesters, carrying wooden crosses and some wearing glow-in-the-dark crucifixes brought another major highway to a halt.

There have been previous flashpoints between Muslims and Christians, who make up about 10 percent of the 80 million population.

Field Marshal Mohamed Hussein Tantawi, head of the ruling military council that is governing the country after Hosni Mubarak quit, said the army would rebuild the church before the Easter holidays. (© Reuters 2011 03/08/11)