Hezbollah leader Imad Moughniyah, on the United States' most wanted list for attacks on Israeli and Western targets, was killed by a bomb in Damascus, the Lebanese group said on Wednesday.
Hezbollah swiftly accused Israel of assassinating Moughniyah, who was head of the Hezbollah security network during Lebanon's 1975-90 civil war. In Gaza, Hamas Islamists called for the Arab world to unite against Israel.
Israel denied any involvement in the killing, seen as a blow to Syrian-backed Hezbollah that fought a 34-day war with Israel in 2006.
Moughniyah, 45, was killed late on Tuesday by a bomb planted in his car. He had long been on a list of foreigners Israel wanted to kill or apprehend and the United States had offered a reward for his capture.
He was implicated in the 1983 bombings of the US embassy and US Marine and French peacekeeping barracks in Beirut, which killed over 350 people, as well as the 1992 bombing of the Israeli embassy in Buenos Aires and the kidnapping of Westerners in Lebanon in the 1980s.
The United States indicted him for his role in planning and participating in the June 14, 1985, hijacking of a US TWA airliner and the killing of an American passenger.
Hezbollah, a group backed by Syria and Iran, announced the assassination and called followers to his funeral on Thursday.
"After a life full of jihad, sacrifices and accomplishments ... Haj Imad Moughniyah ... died a martyr at the hands of the Israeli Zionists," said Hezbollah, which fought a 34-day war in 2006 with the Jewish state.