CAIRO (Reuters) - Hundreds of Muslims have set fire to homes of Christians in southern Egypt and stripped a 70-year-old woman naked after rumours her Christian son had an affair with a Muslim woman, the local church and witnesses said.

The Christian man fled with his wife and children on May 19, said Ishak Ibrahim at the Egyptian Initiative for Personal Rights. His parents went to the police, fearing for their lives.

The next day, around 300 Muslim men set fire to and looted their house in the southern province of Minya and stripped the mother naked out on the street. They also set fire to and looted six other houses, eyewitnesses told Reuters.

"They burned the house and went in and dragged me out, threw me in front of the house and ripped my clothes. I was just as my mother gave birth to me and was screaming and crying," the woman, who requested anonymity, told Reuters.

President Abdel Fattah al-Sisi condemned the attack in a statement on Thursday and ordered authorities to bring those behind it to justice. He also ordered local authorities and the military to rebuild all damaged properties within a month at state expense.

Coptic Orthodox Pope Tawadros II called for calm and restraint in a statement on Thursday. He said he was pursuing the matter with Egyptian officials and that he had spoken to the woman and all those whose homes were attacked.

The woman accuses three Muslim men of stripping her and dragging her in front of her house, her lawyer Ehab Ramzi told Reuters.

Security sources said police arrested five men in connection with the incident and the public prosecutor had ordered their detention and the arrest of 18 others.

Ten members of parliament put forward a motion to cross-examine Interior Minister Magdi Abdel Ghaffar over the incident.

Orthodox Copts, who make up about 10 percent of Egypt's 90 million people, are the Middle East's biggest Christian community. They have long complained of discrimination under successive Egyptian leaders. 

(Reporting by Mohamed Abdellah and Ahmed Aboulenein; Additional reporting by Ali Abdelaty; Writing by Ahmed Aboulenein; Editing by Tom Heneghan)