The head of Hizbollah threatened on Thursday to strike at Israeli targets abroad, in what he called an “open war” provoked by the killing of one of the Shia militant group’s top commanders.

In a chilling speech broadcast to supporters, an emotional and angry Sayyed Hassan Nasrallah told those gathered in the southern suburbs of Beirut for the funeral of Imad Moughniyah that the assassination would only bolster the movement’s resolve.

Hinting that Hizbollah’s retaliation could start within days, he warned that the killing of Moughniyah in the Syrian capital, Damascus, on Tuesday night had changed the parameters of the conflict with Israel, taking the fight beyond Lebanon’s borders.

“You have killed Hajj Imad outside the natural battlefield,” said Mr Nasrallah to an audience that included Manouchehr Mottaki, Iran’s foreign minister. “With this murder, its timing, location and method – Zionists, if you want this kind of open war, let the whole world listen: Let this war be open.”

Israel has denied involvement in the Damascus car bombing that took the life of one of the world’s most wanted men, accused by Washington of a string of attacks in Lebanon in the 1980s.

But with Hizbollah leaders firmly blaming it, much of the country was on high alert. Jewish and Israeli institutions abroad were also told to tighten security, in anticipation of attacks by the Lebanese group.

Israeli officials said the stridency of Mr Nasrallah’s remarks showed how rattled the Hizbollah leader was. “He is in a tight spot,” one said.

Some analysts, meanwhile, said the killing could strain Hizbollah’s relations with Syria, as questions would be raised over how Moughniyah’s safety was compromised.

Mr Nasrallah insisted that the killing was a continuation of the 2006 summer war, in which Hizbollah stood up to a month-long Israeli offensive.

Adopting some of his strongest language towards Israel, Mr Nasrallah said the blood of Moughniyah would “hopefully erase them [Israel] from existence”. His speech was occasionally interrupted by chanting from the crowd.

The slain commander’s importance to Iran – which is, along with Syria, a key backer of Hizbollah – was also underscored when Mr Mottaki read out a letter of condolence from President Mahmoud Ahmadi-Nejad. Iran’s supreme leader, Ayatollah Ali Khamenei, hailed Moughniyah as a “great man”.

Mr Nasrallah also fired a warning shot across the bow of his pro-western Lebanese political opponents, warning them that: “Lebanon will never be Israel, Lebanon will never be American. Lebanon will remain a country of resistance.”

Hizbollah, the most powerful group in the Lebanese opposition, has been in a power struggle with pro-western government parties, leaving the country without a president since November and with a paralysed parliament.

Pro-government groups say they are resisting Hizbollah’s attempt to sweep Lebanon into the “Syrian-Iranian axis”. Hizbollah has vowed that the country will not fall under US influence.

Earlier in the day government supporters had gathered to mourn their own martyr, Rafiq Hariri, the former prime minister, who was assassinated in Beirut in 2005, a killing they blame on Damascus although Syria has always denied involvement.

Hariri and Moughniyah, however, had little in common.

The Hizbollah commander was wanted in many parts of the world and was a symbol of the group’s struggle with Israel. Hariri was an internationally admired billionaire businessman and architect of Lebanon’s reconstruction after its 1975-1991 civil war, and his death became a symbol of Lebanon’s struggle with Syria.

While Hizbollah’s leadership depicted Moughniyah on Thursday as a national hero, the mood among the group’s opponents was strikingly different.

“Who is this Moughniyah, I’ve never heard of him,” said a young supporter of a pro-government group. “He’s not a national figure, he was a criminal.”