A bomb wounded two Egyptian policemen in Cairo and security forces quickly dispersed small protests on Sunday, the anniversary of a popular uprising that toppled autocrat Hosni Mubarak in 2011, officials said.
The blast targeted policemen stationed outside a sports club in Cairo's Heliopolis area, security sources said.
In the Nile Delta region of Baheira, about 170 km (104 miles) from Cairo, two militants were killed when bombs they were planting exploded, state television reported.
Supporters of deposed president Mohamed Mursi of the Muslim Brotherhood gathered near Tahrir Square -- symbolic heart of the 2011 revolt -- and held up photographs of him, a Reuters witness said. Security forces rounded them up. They also used teargas to disperse a protest in Cairo's Ramses Square, officials said.
Tensions have been rising in Egypt. A woman protester was shot dead on Saturday near Cairo's Tahrir Square, the symbolic heart of the revolt that ended Mubarak's three decades in power. Dozens of protesters were killed during last year's anniversary.
On Friday, a 17-year-old woman was killed during protests in Egypt's second city Alexandria. The outlawed Muslim Brotherhood accused security forces of shooting her. Her uncle said she had not been protesting but had gone to a store when she was shot.
State news agency MENA said 22 armoured vehicles were parked around Tahrir Square and roads to it were sealed off on Sunday.
Security forces were also dispatched to Rabaa Square in northeast Cairo, where hundreds of Mursi supporters were killed in August 2013 at a protest camp.
A security crackdown has largely ended Egyptian street demonstrations, but several have taken place in the past week in Cairo and Alexandria.
In a televised address on Saturday, President Abdel Fattah al-Sisi praised the desire for change that Egyptians showed four years ago but said it would take patience to achieve all of "the revolution's goals".
Sisi, who as army chief toppled Mursi in 2013 following mass protests against his rule, says his government is committed to democracy. Human rights groups accuse Sisi of restoring authoritarian rule to the most populous Arab state.
Opponents say new laws, including one restricting protests, have rolled back freedoms won in the uprising against Mubarak.
An Egyptian court ordered the release of Mubarak's sons Alaa and Gamal on Thursday pending a retrial in a corruption case. Last November, a court dropped charges against Mubarak of conspiring to kill protesters in the uprising.