BENGHAZI, Libya (Reuters) - A strong explosion rocked the Libyan military intelligence offices in the eastern city of Benghazi on Wednesday but caused no casualties, security sources and eyewitnesses said.

The blast occurred early on Wednesday when attackers threw an explosive device at the building from a passing car, security sources in Benghazi said. The explosion was so strong that it shattered windows in nearby buildings on Dubai boulevard.

Adil Othmane, an army spokesman in Tripoli, confirmed the incident and said it was under investigation. He attributed the attack to the absence of security guards.

It was the third time the building had been attacked this year. For many Libyans in Benghazi, where the revolt that toppled long-time ruler Muammar Gaddafi began, the building symbolizes the oppression they endured at the hands of his government.

Benghazi has seen a spate of attacks, as well as protests by local groups who complain that eastern Libya is being neglected by the authorities in the capital Tripoli and who want more powers for the region.

Seven Iranian relief workers, official guests of the Libyan Red Crescent Association, were abducted on Tuesday in the heart of the city by an unknown armed group.

(Reporting By Mohamed al-Tommy in Benghazi and Ali Shuaib in Tripoli, Writing by Souhail Karam, editing by Tim Pearce)