France's foreign minister said it was likely that Assad's government had a hand in the assassination. Laurent Fabius told Europe-1 radio that while it was not fully clear who was behind the attack, it was "probable" that Syria played a role.
"Everything suggests that it's an extension of the Syrian tragedy," he said.
Dozens of anti-Syrian protesters erected eight tents near the Cabinet headquarters in central Beirut, saying they will stay until Prime Minister Najib Mikati's government, which is dominated by the Shiite militant group Hezbollah and its allies, resigns. Hezbollah is Syria's most powerful ally in Lebanon, which for much of the past 30 years has lived under Syrian military and political domination.
"The Syrian regime started a war against us and we will fight this battle until the end," said protester Anthony Labaki, a 24-year-old physiotherapist who is a member of the right-wing Phalange Party. He said the protesters will not leave the area until Mikati's government resigns and those behind al-Hassan's killing are uncovered.
Syria's hold on Lebanon began to slip in 2005, when former Prime Minister Rafik Hariri, an opponent of Syria, was assassinated in truck bomb along Beirut's Mediterranean waterfront. Syria denied any role. But broad public outrage in Lebanon expressed in massive street protests forced Damascus to withdraw its tens of thousands of troops from the country.
For years after the pullouts, there was a string of attacks on anti-Syrian figures in Lebanon without any trials for those responsible. Assad has managed to maintain his influence in Lebanon through Hezbollah and other allies.