CAIRO: A bomb exploded near a bus in Cairo Thursday injuring five people, a day after the government widened a crackdown on Islamists by declaring the Muslim Brotherhood a terrorist group.

The windows of the red and black public transport bus were shattered in the explosion at a busy intersection in the north Cairo neighbourhood of Nasr City.

Police defused a second bomb and cordoned off the area, an AFP correspondent said.

The bombing came a day after the military-installed government declared the Muslim Brotherhood, the movement of deposed president Mohamed Morsi, a terrorist organisation, and after a suicide car bomber killed 15 people at a police building.

The interior ministry said the attack was meant to intimidate voters ahead of a referendum next month on a new constitution, billed as the first step in a democratic transition ahead of parliamentary and presidential elections.

A police general, Mohamed Gamal, showed a defused pipe bomb to reporters, saying it had been placed in a billboard and was primed to explode when police arrived at the site of the first bombing.

“It was set to go off remotely,” interior ministry spokesman Hany Abdel Latif told AFP, adding the bombs were “meant to terrorise people before the referendum”.

A witness described scenes of panic after the attack.

“I was 100 metres away when I heard the explosion. I came running to help the wounded,” said one witness, Mahmud Abd al-Al, a construction worker.

“They were covered in blood. One man lost a leg,” he said.

A health ministry official said five people were wounded in the attack.

The Brotherhood, which organises near daily protests demanding Morsi's reinstatement, insists it is peaceful and has condemned militant attacks.

It was declared a terrorist after Tuesday's bombing of a police headquarters that killed 15 people, although an Al Qaeda-inspired group in the Sinai Peninsula said it was responsible.

The bombing in Cairo appears to be the first against civilians, although it could have been meant for another target and exploded prematurely.

A suicide bomber had set off a car bomb in September in the same neighbourhood in an attempt to assassinate interior minister Mohamed Ibrahim as his convoy passed by.

The minister survived but a bystander was killed in the explosion.

Militants have killed scores of policemen and soldiers in attacks mostly in the Sinai Peninsula since the military's overthrow of Morsi in July.

The deadliest attacks, including Tuesday's bombing in the city of Mansoura, have been claimed by the Al Qaeda-inspired Ansar Beit al Maqdis group based in the restive Sinai.

The group which is composed mostly of Egyptian Bedouin has been critical of the Brotherhood's style of political Islam and advocates armed attacks.

Authorities say there are links between the Sinai jihadists and Morsi's more moderate Muslim Brotherhood movement, but have offered no proof.

Morsi and top Brotherhood leaders, imprisoned in a crackdown following his overthrow, are charged with colluding with militant groups to launch attacks in the country.

The drastic decision to list the 85-year-old movement as a terrorist group means their vast grass roots and charity networks will be targeted and seized by the state.

The cabinet had come under increased pressure to declare the Brotherhood a terrorist group following Tuesday's bombing.

Morsi's year in power, after the country's first democratic presidential elections, alienated many Egyptians who accused the Islamists of trying to monopolise all branches of government.

Since his ouster, more than 1,000 people, mostly Islamists, have been killed in a police crackdown and thousands imprisoned.