Many were injured in the second blast after they rushed to the area to help the casualties of the first explosion in the working class Gungoren neighborhood, witnesses said. The blasts were about 10 minutes apart.

"There is no doubt that this is a terror attack," Gov. Muammer Guler told reporters. "The fact that there was a crowd in the area has increased the number of casualties," he added.

The governor's office said 154 people were injured. Of those, 15 were in critical condition, a government official at the scene said. Police were investigating who was behind the blasts.

CNN-Turk television, citing security sources, said police suspect Kurdish rebels may be responsible for the attack. It said intelligence reports had suggested the rebels were planning a bombing campaign in Turkish cities.

However, officials did not accuse any specific group.

"We know it is a terrorist attack, but which organization is responsible - we don't yet have that information," Deputy Prime Minister Hayati Yazici told journalists at the scene of the attacks.

Kurdish, leftist and Islamic militants are active in Istanbul and have carried out past bombings in the city. On July 9, gunmen fired on police guarding the U.S. Consulate in Istanbul, killing three officers. Three attackers also died in a shootout with police. Police were looking into possible links with al-Qaida or whether the gunmen were operating on their own.

In 2006, a bomb targeting police officers blew up at an Internet cafe near the riot police headquarters, killing one person and wounded 16 others.