Three small bombs exploded on Sunday in a western Cairo suburb, targeting an electricity transmission pylon and cutting power to the area for nearly an hour, a security official said.

Militants have stepped up attacks in Egypt, mostly against the security forces, since the army ousted Islamist president Mohammed Morsi in July 2013 and the new authorities launched a deadly crackdown on his supporters.

Four suspected Egyptian Islamists were killed south of Cairo early Friday while preparing explosives on a farm owned by a senior member of the banned Muslim Brotherhood, police said.

The blast in a village in the rural Fayoum province came hours after nine people were wounded when a small bomb went off in a train in the Mediterranean city of Alexandria.

A limited explosion occurred late Wednesday inside a car near a Cairo military hospital, Egypt's interior ministry said, hours before the first anniversary of deposed president Mohamed Morsi's military ouster.

Police arrested one of two men who were in the car while the second managed to escape, interior ministry spokesman Hani Abdel Latif told Agence France Presse, adding that the cause of the blast was still unknown and there were no casualties.

Two bombings on Saturday in a Cairo suburb killed a teenager and her mother, officials said, the latest in a wave of blasts to hit the Egyptian capital this week.

Militants have stepped up attacks after the army ousted Islamist president Mohammed Morsi in July 2013 and amid a deadly crackdown by authorities on his supporters.

Four makeshift bombs exploded within minutes of each other at three Cairo metro stations during morning rush hour on Wednesday, wounding at least two people, Egyptian police said.

One bomb went off at the station of Ghamra, in central Cairo, while the others occurred at Shubra el-Kheima and Hadayek al-Kobba on the outskirts of the capital, a police official told Agence France Presse.

A trip to Egypt is currently not on the agenda of Prime Minister Tammam Salam, officials said in remarks published on Wednesday.

The officials, who are close to Salam, told al-Liwaa newspaper that Salam is not planning to visit Cairo anytime soon although he is ready to accept official invitations that he receives from Arab and friendly countries.

Gunmen travelling in a car opened fire on Tuesday on a group of Egyptian policemen outside Cairo's Al-Azhar university, killing three and wounding nine others, the interior ministry said.

The attack comes just days ahead of a presidential election on May 26-27, which former army chief Abdel Fattah al-Sisi is expected to win.

An Egyptian court on Sunday sentenced 126 supporters of ousted Islamist president Mohammed Morsi to 10 years in prison each in a mass trial over protest violence, judicial sources said.

Another court, in Cairo, sentenced a further 37 alleged Morsi supporters to 15 years in prison for trying to set off bombs during a protest at a metro station last year.

A bomb explosion wounded four people overnight at a rally in Cairo for presidential election frontrunner Abdel Fattah al-Sisi, Egyptian security officials said on Sunday.

Two policemen were among those hurt in the attack, the first targeting a rally since campaigning began two weeks ago for the May 26-27 election, said the officials.

Hailed as a savior for overthrowing an Islamist president, former army chief Abdel Fattah al-Sisi can count on the vote of Egypt's Coptic Christians who view him as a bulwark against fundamentalists.

The Copts are the Middle East's largest religious minority, and have long suffered sectarian violence that culminated in attacks on churches by supporters of ousted Islamist president Mohamed Morsi last year.