Many Lebanese were in a state of shock and disbelief on Tuesday evening, unable to get their heads around an event that was unprecedented in scale and nature.

Hezbollah said an unspecified number pagers – which the group relies on heavily for communications due to the risk of mobile phones being hacked or tracked – exploded at around 15:30 local time (12:30 GMT) in the capital Beirut and many other areas.

One CCTV video showed an explosion in a man’s bag or pocket at a supermarket. He is then seen falling backwards to the ground and crying out in pain as other shoppers run for cover.

Hours later, ambulances were still rushing to hospitals overwhelmed with the number of casualties, 200 of whom the health minister said were in a critical condition. Outside, relatives were waiting in the hope of receiving updates.

The LAU Medical Centre in Beirut’s Ashrafieh district closed its main gate and was limiting the number of people getting in. “It’s very sensitive and some scenes are horrific,” one staff member told the BBC.

Most of the wounds were at the level of the waist, face, eyes and hands, he said, adding: “A lot of casualties have lost fingers, in some cases all of them.”

The wife of Iranian ambassador Mojtaba Amani said he was “slightly injured” by one of the explosions and that he was “doing well” in hospital.

Hezbollah’s media office announced the deaths of eight fighters. It did not give details on the locations and circumstances, saying only that they were “martyred on the road to Jerusalem”.

A source close to the group told AFP news agency that the son of Hezbollah MP Ali Ammar and the 10-year-old daughter of a Hezbollah member in the Bekaa Valley were among those killed. Later, the source said the son of another lawmaker, Hassan Fadlallah, was wounded, having initially reported that he was dead.

Fourteen people were also wounded by exploding pagers in neighbouring Syria, where Hezbollah is fighting alongside government forces in the country’s civil war, according to the UK-based Syrian Observatory for Human Rights.

“We hold the Israeli enemy fully responsible for this criminal aggression,” Hezbollah said in a statement on Tuesday evening.

“This treacherous and criminal enemy will certainly get his just retribution on this sinful aggression from where it counts and from where it does not count,” it added.

Lebanese Prime Minister Najib Mikati also blamed Israel for the explosions, saying that they represented a “serious violation of Lebanese sovereignty and a crime by all standards”.

Iranian Foreign Minister Abbas Araghchi said he told his Lebanese counterpart that he “strongly condemned Israeli terrorism”.

The US, Israel’s closest ally, denied any involvement and urged Iran not to heighten tensions.

Hezbollah did not say what it believed had caused the pagers to explode.

The Wall Street Journal cited a source as saying the affected devices were from a new shipment that Hezbollah had received in recent days. A Hezbollah official also told the newspaper some people had felt the pagers heat up before the blasts.

Overheated lithium-ion batteries can catch fire, but experts said hacking into the pagers and making them overheat would not usually cause such explosions.

A former British Army munitions expert, who asked not to be named, told the BBC the pagers would have likely been packed with between 10g and 20g of military-grade high explosive, hidden inside a fake electronic component.

Once armed by a signal, called an alphanumeric text message, the next person to use the device would have triggered the explosive, the expert said.

Lina Khatib, a Middle East analyst at the UK-based Chatham House think tank, told the BBC: “Israel has been engaging in cyber operations against Hezbollah for several months, but this security breach is the largest in scale.”

Nicholas Blanford, a Beirut-based senior fellow of the American think tank the Atlantic Council, said: “Israel in one fell swoop has rendered combat ineffective hundreds if not thousands of Hezbollah fighters, in some cases permanently.”

He warned that Hezbollah’s leaders would now “face extreme pressure from the ranks and supporters to retaliate heavily”, describing it as “the most dangerous moment” in the Hezbollah-Israel conflict since October.