CAIRO (REUTERS) - A wave of bomb attacks targeting police hit Cairo on Friday, killing six people on the eve of the third anniversary of the uprising that toppled autocrat Hosni Mubarak and raising fears that an Islamist insurgency is gaining pace in Egypt.

The violence underscored the struggle of authorities to tame militant violence which has increasingly challenged the state since the army toppled Islamist President Mohamed Mursi in July.

In the most high-profile attack, a car bomb exploded at a security compound in central Cairo early in the morning and killed at least four people, including three policemen, security sources said.

They said the blast was the work of a suicide bomber. But footage broadcast on an Egyptian television channel showed a man getting out of a van and moving into another vehicle. Minutes later the van exploded.

Another blast in the Dokki district killed one person. An explosion near a cinema on the road to the Pyramids of Giza on the outskirts of Cairo also led to one fatality.

Clashes in the capital and several other cities between Mursi supporters and security forces which killed 11 people also raised tensions in the biggest Arab nation.

There was no immediate claim of responsibility for the car bomb attack in the parking lot of the Cairo Security Directorate, or the other blasts. But they had all the trademarks of attacks carried out by militants seeking to topple the army-backed government.

Prime Minister Hazem el-Beblawi condemned the bloodshed in a statement, saying it was an attempt by "terrorist forces" to derail the army-backed government's political road map, which is meant to lead to free and fair elections.

Later in the day, a military helicopter flew back and forth over central Cairo, underscoring concerns that another attack could occur at any time.

In Washington, the White House condemned the bombings and urged all sides to avoid violence. "These crimes should be investigated fully and the perpetrators should be brought to justice," spokesman Jay Carney told reporters.

Authorities have been bracing for more violence during the anniversary of Mr Mubarak's fall, when rival political groups are expected to turn out, including supporters of army chief General Abdel Fattah al-Sisi, who ousted Mursi, as well as members of the Muslim Brotherhood and liberals.

The 2011 revolt raised hopes of a stable democracy in the Arab world's biggest nation. Instead, relentless political turmoil has hit investment and tourism hard in Egypt.

In a statement, the office of President Adly Mansour said it would "avenge the deaths of the martyrs" who died at the Security Directorate and severely punish the perpetrators.