French police detained Muhammad Zuhair al-Siddiq, a witness in a UN inquiry into the February killing, on Sunday on an international warrant.
Lebanese judicial sources said they had asked for al-Siddiq's detention on murder charges because they believed he had an indirect role in al-Hariri's killing and had misled international investigators.
Al-Siddiq faces the same charges as four pro-Syrian Lebanese generals detained since August on the recommendation of chief UN investigator Detlev Mehlis and charged with murder, attempted murder and carrying out a terrorist act in connection with the assassination, the Lebanese judicial sources said.
Lebanon has asked that al-Siddiq be extradited but was awaiting a French decision on the issue, they added.
French judicial sources said on Monday Beirut had 30 days to provide the necessary documents for the extradition request.
When he presents his report to the United Nations this week, Mehlis is expected to implicate Syrian officials in an assassination that plunged Lebanon into its worst security crisis since the 1975-1990 civil war.
It stirred local protests and international pressure that led ultimately to the withdrawal of Syria from Lebanon where it had long played a power-broking role.
Lebanese political sources say al-Siddiq was one of the leading witnesses in the inquiry, having said he attended meetings at which al-Hariri's killing was discussed, but became a suspect when it transpired he had misled investigators.
Interior Minister Ghazi Kanaan is
said to have committed suicide
Lebanese newspapers reported that suspicions had been raised when al-Siddiq told investigators he was nearby when the bomb blast that killed al-Hariri and 20 others went off.
Last week, Syrian Interior Minister Ghazi Kanaan was found dead in his office, his apparent suicide coming three weeks after he was questioned by the UN team investigating al-Hariri's death.
Syrian President Bashar al-Assad said in a CNN interview conducted shortly before Kanaan's death that his country was not involved in al-Hariri's death and that he could never have ordered the murder.
Should the UN conclude Syrians were involved, the people so implicated would be "traitors" who would face an international court or Syrian justice, he said.
UN spokesman threatened
The spokesman for the UN investigation into al-Hariri's assassination has meanwhile been pulled out of Lebanon after a threat against him, the UN said on Tuesday.
Nejib Friji, the head of the UN Information Centre in Beirut, was temporarily taken out of Lebanon for his own safety, UN chief spokesman Stephane Dujarric said.
The threat "was deemed serious enough for him to be
removed", Dujarric said, but he declined to elaborate on its
precise nature or timing.
Friji, a Tunisian, has now been reassigned to Peter van Walsum, UN Secretary-General Kofi Annan's special envoy for Western Sahara.