The youngest son of Muammar Gaddafi was killed in a NATO airstrike on a house where the Libyan leader and his famliy were staying at the time, a government spokesman said Sunday.

The raid resulted in the deaths of Saif al-Arab Gaddafi, 29, the youngest son of Gaddafi, and three of Gaddafi's grandchildren, spokesman Mossa Ibrahim told a press conference.

The Libyan leader himself and his wife, who were in the house at the time, were in good health, while some other people were injured in the attack, Ibrahim said.

This was a direct act to assassinate the Libyan leader, a violation of international law which has no legal or political justifications, he said.

After the raid which took place around 8:00 p.m. local time (1800 GMT) Saturday, Ibrahim took a group of journalists to the site of of the house, that is located in a residential area in western Tripoli.

Hit by three missiles, the three-storey building was partially destroyed, with roofs completely caved in some portions, leaving mangled rods of reinforcing steel hanging down among splintered chunks of concrete.

While two of the missiles had exploded, the third was seen lying in one of the rooms of the building.

After the air strike, large numbers of Libyans took to the streets, protesting the NATO strikes against civilians and expressing their support to Gaddafi's family.

Prior to the raid, Gaddafi floated on Saturday a proposal for a cease-fire and negotiations in a televised speech while asserting that he won't leave the country.

He said all parties concerned should follow the truce, and the NATO forces must stop their attacks.