Explosions have been heard across central Cairo, with some reports suggesting blasts at the state security building and a courthouse in a northern suburb of the Egyptian capital.
The Reuters news agency reports that witnesses heard several blasts from the Shubra al-Khaima area of the city. There are no official reports of casualties yet.
Reuters also report that one Egyptian security official believes it was a car bomb.
A picture purporting to show the beheaded body of a Croatian man held hostage by Islamist militants in Egypt have appeared on Twitter.
The SITE monitoring service said the Sinai Province extremist group - which has pledged support to the so-called Islamic State - had threatened to kill Tomislav Salopek last week unless Muslim women being held in Egyptian jails were not freed.
The image has not been independently verified.
A spokesman from the Egyptian Interior Ministry said the authorities were making their own checks and would reveal more in due course.
A court in Egypt has postponed the verdict in the retrial of Al Jazeera television journalists who have been charged with aiding a "terrorist organisation" for the second time.
The verdict hearing has been postponed until 29th August.
Mohamed Fahmy, a naturalised Canadian who has given up his Egyptian citizenship, and Egyptian Baher Mohamed were released on bail in February after spending more than a year in custody.
Both men deny all charges.
A third Al Jazeera journalist, Australian Peter Greste, was deported in February.
At least 18 people have been killed after a cargo boat collided with a passenger boat on Egypt's river Nile.Read the full story ›
Islamic State's Egypt branch, known as Sinai Province, has claimed to have carried out a rocket attack on an Egyptian naval vessel in the Mediterranean Sea.
The Egyptian military admited that a coastguard vessel had exchanged shots with militants off the coast of northern Sinai, an area bordering Israel and the Gaza Strip. The boat caught fire but the incident resulted in no casualties, the military said.
A security official and Egypt's state news agency MENA said a powerful explosion in Cairo occurred in front of the Italian consulate in the city centre.
A car bomb caused the blast, the security official said. There was no immediate claim of responsibility for the explosion. The MENA report cited witnesses as saying the force of the explosion had caused massive destruction to the building.
A huge explosion has rocked central Cairo, a witness for the Reuters news agency has reported.
Local media say that blast happened outside the Italian consulate, and it is thought to have been a car bomb.
In recent months, Islamist militants seeking to topple the government have claimed responsibility for roadside bombs and suicide attacks across Egypt.
At least 20 members of the Egyptian security forces have been killed in suspected militant attacks in the country, military officials have said.
A police station and five checkpoints were targeted in North Sinai, with a car bomb exploding at one military post.
An army spokesman said 22 of the attackers were killed during the assaults.
A Sinai-based insurgency has killed hundreds of policemen and soldiers since the Egyptian army toppled Islamist president Mohamed Mursi after mass protests against his rule in 2013.
A suicide bomber has blown himself up near a tourist site in the Egyptian city of Luxor, security sources and witnesses say.
Three assailants were killed in the attack, which took place close to the ancient Karnak temple, one of Egypt's most popular tourist destinations.
Two gunmen were shot dead by police, but a third person managed to pass through a barrier leading to the site before blowing himself up.
Four Egyptians - including shop owners and police officers - were wounded in the attack.
No group immediately claimed responsibility.
Egypt's former president Hosni Mubarak is to be retried over the killing of protesters in 2011.
Last year Mubarak was cleared in a retrial after originally being jailed for ordering the killing of protesters during the uprising in 2011 that ended with his removal from power.
His retrial will begin in November, Egypt's high court ruled.