BEIRUT—Pagers carried by hundreds of Hezbollah operatives exploded over a short period Tuesday, leaving many of them injured in an unprecedented event that struck across Lebanon.

The affected pagers were from a new shipment that the group received in recent days, people familiar with the matter said. A Hezbollah official said hundreds of fighters had such devices, speculating that malware may have caused the devices to heat up and explode. The official said some people felt the pagers heat up and disposed of them before they burst.

It couldn’t immediately be determined what caused the blasts, which were spread out across the country in several areas where Hezbollah has a heavy presence. The Israeli military declined to comment.

The number of casualties was rising too quickly to count but appeared to top 1,000, a Hezbollah official said. The group seemed overwhelmed trying to keep up, and hospitals across the country struggled to treat the injured. Downtown Beirut was filled with the sirens of ambulances wailing by.

Lebanon’s Internal Security Forces said certain types of wireless communication devices had exploded in various parts of the country, with a heavy concentration in Beirut’s southern suburbs. They called on citizens to clear the roads to facilitate the transportation of the injured to hospitals.

Wounds included severed fingers, head injuries and large gashes to people’s torsos.

Lebanese Health Minister Firas Abiad said he needed more time to determine the total number of casualties but assessed that the number of emergency room admissions in southern Lebanon, a Hezbollah stronghold, was extremely high.

“Clearly the number will be in the hundreds of casualties,” Abiad said. “A lot of patients are in the emergency sections of hospitals in most parts of the country, and the health apparatus is working on triaging these cases.”

He said he didn’t know how and why this happened, adding that many medical staff had got rid of their pagers fearing a second wave of detonations.

The incident comes amid growing tension along Lebanon’s border with Israel. Hezbollah began firing rockets into Israel soon after the Hamas-led Oct. 7 attacks on southern Israel that set off the war in Gaza, and the two sides have exchanged fire daily since then, driving tens of thousands of people out of towns on both sides of the border and leaving hundreds of Hezbollah operatives dead.

An ambulance vehicle drives as hundreds of members of the Lebanese armed group Hezbollah, including fighters and medics, were seriously wounded on Tuesday when the pagers they use to communicate exploded, according to a security source, in Sidon, Lebanon September 17, 2024. REUTERS/Aziz Taher

Write to Summer Said at summer.said@wsj.com and Stephen Kalin at stephen.kalin@wsj.com