The hearing took place against the backdrop of a political crisis in Lebanon, where the Hezbollah movement and its allies toppled the government of Hariri's son, Saad Hariri, over his refusal to cut links with the tribunal.

"This hearing signals an important moment for the life of the Special Tribunal for Lebanon," presiding Judge Antonio Cassese said, adding this was the start of proceedings for the court, which has faced repeated criticism over its mandate.

The Lebanon tribunal is the first international court with jurisdiction over the crime of terrorism, but is grappling with how to apply Lebanese and international law before including a terrorism charge in potential arrest warrants.

It was set up to try those accused over the 2005 bombing that killed Hariri and 22 others, and prosecutor Daniel Bellemare sent a still-sealed indictment on Jan. 17 to pre-trial judge Daniel Fransen.

The indictment is expected to accuse Hezbollah members of involvement in the killing, but Hezbollah denies any involvement and has warned anyone against taking action against its members.

Fransen has posed to court judges 15 questions dealing with definitions of terrorism, conspiracy and homicide and other legal issues such as criminal responsibility and multiple charges, before making a decision on any arrest warrants.

The court will make a ruling on Feb. 16.

Experience at the International Criminal Tribunal for the former Yugoslavia has shown the difficulty in linking crimes to leaders, giving importance to an agreement on how to apply notions of a conspiracy and a "joint criminal enterprise."

Prosecutor Daniel Bellemare also said there were no gaps in Lebanese law on terrorism, and there was no need for political intent to be involved for a crime to be defined as terrorism.

Alia Aoun, deputy head of the defense office, urged against agreeing firm definitions, saying this would prematurely impose them on trial judges, limiting the rights of the accused.