Twin homemade bombs targeting a police checkpoint have wounded six people on a bridge in Cairo's twin city of Giza.
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 upmarket 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 neighboring 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.
Interim Prime Minister Hazem Biblawy condemned the bombing. He said that "terrorist" attacks will not deter Egyptians from taking "steps towards their future."