CAIRO, Egypt -- Twin homemade bombs targeting a police checkpoint wounded six people on a bridge in Cairo's twin city of Giza Friday morning, the latest apparent attack by militants near the centre of the Egyptian capital.

The state news agency MENA quoted an Interior Ministry spokesman as saying the explosions hit a truck belonging to the Central Security forces that was stationed on the bridge leading into a major square.

A security official said four of the wounded are police.

The blasts came amid heightened security presence in the capital in preparation for another Friday of demonstrations by Muslim Brotherhood supporters around the country against the military-backed interim government.

Security was beefed up around the presidential palace in the upscale district of Heliopolis in anticipation of possible violence.

The Muslim Brotherhood-led coalition that supports ousted president Mohammed Morsi, who was removed from power in July's coup, issued a call earlier this week to "continue the revolution." Pro-Morsi protesters have staged near-daily demonstrations since his ouster with the largest rallies usually on Friday or key anniversaries, though their numbers have dwindled in the recent months amid a security crackdown.

The group has also planned a series of marches in the run up to Tuesday, the third anniversary of the day on which long-time autocrat Hosni Mubarak stepped down following the country's 2011 uprising.

Scores of protesters marched in Cairo and the neighbouring provinces of Giza and Fayoum. In one march in the capital's eastern district of Nasr City, police fired tear gas to disperse the demonstrators chasing them into side streets, security officials said.

Officials spoke on condition of anonymity because they were not authorized to brief reporters.

In a statement on MENA, interim Prime Minister Hazem Biblawy condemned Friday's bombing. He said that "terrorist" attacks will not deter Egyptians from taking "steps towards their future."

Egypt has seen a spike in attacks since Morsi's overthrow, often targeting security forces. Many have been claimed by a Sinai-based al Qaeda-inspired militant group.

Last week, a new group calling itself Ajnad Misr, Arabic for Egypt's Soldiers, claimed responsibility for several such bombings.