Breaking News: Big explosion hits Cairo police headquarters, killing at least 4.

A plume of black smoke rose over Cairo early Friday after a powerful explosion hit the city's police headquarters, killing at least four people and wounding more than 50 others, Egyptian authorities said. The blast struck a key symbol of authority in a country that has been shaken in recent years by political turmoil and violent unrest. It was followed by two smaller explosions near police stations in other areas of Cairo, one of which wounded several people. The blasts took place at time of high tension, the day before the third anniversary of the 2011 revolution that eventually brought down the country's former authoritarian leader Hosni Mubarak. And they come amid the instability ushered in by the military's overthrow last year of the democratically elected former president, Mohamed Morsy, and the ensuing crackdown by security forces on the Islamist movement that supported him, the Muslim Brotherhood. Suicide bomber suspected The first blast appeared to have been caused by a suicide attacker who tried to drive a vehicle laden with explosives into the police headquarters, said Maj. Gen. Hany Abdel Latif, a spokesman for the Interior Ministry, citing preliminary reports. Guards stationed in front of the headquarters in the Abdeen district of Cairo opened fire at the vehicle, and the explosion went off in the building's vicinity rather than inside, he told state news agency Egynews. The force of the blast destroyed the front of the first and second floors of the eight-story building, he said, and caused material damage on the third floor. Most of the building's windows appeared to have been blown out. Air-conditioning units dangled by cables from the shattered facade. At least 51 people were wounded in the explosion, state-run broadcaster Masriya TV reported, citing the Health Ministry. Visiting the ruined building, Egyptian Interior Minister Mohamed Ibrahim condemned the bombing. "These are nothing but desperate attacks in an attempt to create chaos, but the citizens here will remain resilient," Ibrahim told Masriya TV. He said security forces would ensure that Egyptians would be able to celebrate the anniversary of the revolution Saturday "as if nothing happened today." More explosions follow CNN's Reza Sayah said the blast appeared to be "the most powerful bomb attack that we've seen here in central Cairo in recent memory." Speaking from near the scene of the blast, Sayah said the attack will likely intensify the fight between Egypt's military-backed government and the groups that oppose it. It wasn't immediately clear who was behind that attack. Friday is a holiday in Egypt, so the police headquarters is unlikely to have been as busy as it would have been on a weekday. The blast happened around 6:30 a.m., according to state media. Footage aired by local broadcasters showed significant damage to the front of the building. The street in front of the headquarters was cordoned off. Hundreds of people -- some stunned, some angry -- gathered around the scene, Sayah said. Many of those in the crowd were quick to blame the Muslim Brotherhood, despite no official word on who might be to blame. The explosion appeared to have shattered windows and damaged other parts of buildings several blocks away, he said. A second, much smaller explosion Friday went off near police station in Dokki, a residential area of Cairo, wounding several people, authorities said. And in the Al-Haram district of the city, a homemade bomb exploded in the vicinity of a police station, the semiofficial Ahram Online news outlet reported. There were no immediate reports of casualties from that explosion. Fears of more turmoil In a referendum earlier this month, Egyptian voters overwhelmingly approved a new constitution proposed by the military-backed government, according to the country's electoral commission. But supporters of the now-banned Muslim Brotherhood had boycotted the vote in response to a continuing government crackdown. Rights groups have expressed concern about what they call an increasingly repressive environment in Egypt, where more than 2,200 people have been killed since the ouster of Morsy. Some anti-government groups have resorted to violence. Bomb attacks and shootings against the security apparatus have taken place across the country in recent months -- a situation described by some as a low-level insurgency. The government has blamed the Muslim Brotherhood for many of the attacks, even though the movement has condemned them. The bombing of the Cairo police headquarters suggested the violence is taking on a more serious, high-profile form. "The fear is there's a very difficult and tumultuous phase ahead for this country," Sayah said. Source: CNN.