* Fighting erupts after Christians block highway
* Christians protesting over church burning
CAIRO, March 8 (Reuters) - About 1,300 Christians and Muslims threw stones and Molotov cocktails at each other on Tuesday during a protest over the burning of a church, an Egyptian security source said.
Christians were protesting for the second day over a church in Helwan on the outskirts of Cairo that was set on fire after a row sparked by a relationship between a Christian man and a Muslim woman, witnesses and a security source said.
About 20 people were injured in the latest sectarian flare-up, and five cars were burnt during the clash between the groups, estimated at a total of 1,300 people, the source said.
The fighting erupted up when a group of Christians blocked a main highway and clashed with Muslims who wanted to pass through, a security source said.
"They began fighting because people in their cars wanted to pass through and the protesters had blocked the highway," the security source said.
Witnesses said the army, which came to the scene, fired gunshots in the air during the skirmish to disperse the crowd but was unable to quell the fight.
Such inter-faith relationships often cause tensions in Muslim-majority Egypt, where Christians make up about 10 percent of the country's 80 million people.
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 Easter holidays. (Writing by Marwa Awad; Editing by Matthew Jones)