Several sectarian attacks against Egyptian Christians have taken place after the removal of former president Mohamed Morsi, according to Human Rights Watch (HRW).

The New York-based organisation called for a government investigation "to hold the perpetrators to account, and determine whether the police could have prevented or stopped the violence".

The worst incident took place on 5 July when a mob of local residents beat four Christians to death in Naga Hassan, a village 10 kilometres west of the southern city of Luxor.

The rampage broke out after a Muslim man was found dead and lasted for 18 hours, during which police did nothing, according to at least 20 people who witnessed the violence.

"Egyptian security forces should be on high alert to prevent and halt sectarian violence in the current tense and polarised situation," said Nadim Houry, acting Middle East director at Human Rights Watch.

"Egypt's religious and political leaders should denounce the dangerous escalation of sectarian attacks."

The backlash has taken place since Coptic pope Tawadros, the spiritual leader of Egypt's 8 million Christians, gave his blessing to the removal of Morsi and takeover of the country by the army. A few churches have been set on fire and Muslim Brotherhood leader Mohamed Badie, who is under house arrest, criticised Tawadros for supporting the military intervention.

At least six attacks on Christians have taken place across Egypt since the army takeover, blighting Luxor, Marsa Matrouh, Minya, North Sinai, Port Said and Qena.

"Authorities in Egypt should ensure that prosecutors promptly and impartially investigate allegations of sectarian violence, whether the victims are Christian or Muslim, and bring prosecutions as appropriate," HRW said.

"The Egyptian government should make ending sectarian violence a priority, or risk letting this deadly problem spiral out of control," Houry said. "Prosecutors should thoroughly investigate and prosecute those responsible, including security forces, if they want to show they are capable of preventing future bloodshed."

Sectarian violence against Coptic Christians has increased in recent years. In January 2011 a car bomb exploded outside the Alexandria Coptic Orthodox Church, killing 21 and injuring at least 79.

In the worst post-revolution incident, 28 people, mostly Copts, were killed during a sit-in in front of the Maspero television building in Cairo, where they had gathered to protest against the demolition of a church in upper Egypt.