Iran’s Supreme Leader Ayatollah Ali Khamenei has said Iran and its regional allies will not back down against Israel and called for unity among Muslim nations as he delivered a rare Friday sermon.

Khamenei led prayers at the Imam Khomeini Grand Mosalla mosque in central Tehran in his first public appearance since Iran launched a massive barrage of some 200 ballistic missiles at Israel on Tuesday.

That attack was in retaliation for Israel’s killings of senior Hezbollah, Hamas and Iran’s Islamic Revolutionary Guard Corps (IRGC) figures, including Hezbollah leader Hassan Nasrallah, and escalating attacks in Lebanon.

“The resistance in the region will not back down even with the killing of its leaders,” Khamenei said, calling Iran’s attack on Israel “legal and legitimate”.

“The operations were … in return for the heinous crimes committed by this bloodthirsty criminal entity,” he said.

He said Iran would fulfil its “duty” to allies in a considered manner.

“We will not act irrationally … not act impulsively”, he said, adding that the country would follow decisions “handed down by our political and military leadership”.

Reporting from Tehran, Al Jazeera’s Resul Serdar said the event was being held at a “delicate and intense time”.

Khamenei’s sermon sent a message to Israel that the Iranian authorities “are not hiding, they are not seeking shelter, they are not going underground”, Serdar said.

It was the supreme leader’s first such sermon in more than four years, coming just before the first anniversary of Hamas’s October 7 attack on Israel, which led to a war that has so far killed more than 41,700 Palestinians and recently spilled over into Lebanon.

Iran’s proxies in its “axis of resistance” – Hezbollah, Yemen’s Houthis and armed groups in Iraq – have carried out attacks in the region in support of the Palestinians in the Gaza war.

Addressing massive crowds, Khamenei issued a rallying call to Muslim nations –  “from Afghanistan to Yemen, from Iran to Gaza and Lebanon” – saying they should unite against common “enemy” Israel, which he claimed had deployed “psychological”, “economic” and “military” warfare against them.

“Our enemy is one,” he said. “If their policies are sowing the seeds of division in one country, they may prevail and once they seize control of one country, they move to the other.”

Al Jazeera’s Serdar said that the message of unity countered “criticism over the past decade” that Iran had been isolating itself from the region.

“His speech was focused on unity because he has seen now that the possibility of a regional war is real and that’s why he is asking Muslims to be united, to somehow eliminate this threat as a common action, so a regional war can be aborted.”

Khamenei last led Friday prayers after the United States killed revered general Qassem Soleimani in Baghdad in 2020.

His speech on Friday was preceded by a commemoration for Nasrallah, killed last week in the southern suburbs of Beirut in an Israeli strike, alongside Abbas Nilforoushan, a general from the Islamic Revolutionary Guard Corps.

On Tuesday, Israel launched a ground offensive in southern Lebanon, an expansion of the war that has seen it repeatedly bomb Beirut and its southern suburbs.

Later that same day, Iran made a retaliatory attack on Israel, its second this year. In April, it had sent a volley of missiles following a deadly Israeli strike on Iran’s consulate in Damascus.

In both attacks, nearly all missiles were intercepted by Israel or its allies, according to Israeli authorities.

Early on Friday, Israel hit Beirut with a barrage of attacks reportedly targeting senior Hezbollah figure Hashem Safieddine, a putative successor to Nasrallah.

There was no comment from Israel or Hezbollah on his fate.

Tehran has told the US via an intermediary that any Israeli attack against Iran would meet an “unconventional response” that includes targeting infrastructure, according to an Iranian official who spoke to Al Jazeera.

US President Joe Biden said on Thursday that Israel’s response could include a strike on Iran’s oil facilities.