There was no immediate comment from the Israeli military on the wave of explosions, which came just hours after Israel announced it was broadening the aims of the war to include its fight against Hezbollah along its border with Lebanon.
The sons of Hezbollah lawmakers Ali Ammar and Hassan Fadlallah were among the dead.
The blasts "killed nine people, including a girl", Lebanese Health Minister Firass Abiad said in a casualty update.
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He added that some "2,800 people were injured, about 200 of them critically" with injuries mostly reported to the face, hands and stomach.
The 10-year-old daughter of a Hezbollah member was killed in east Lebanon's Bekaa Valley when his pager exploded, the family and a source close to the group said.
Tehran's ambassador to Beirut was also wounded in a pager explosion but his injuries were not serious, Iranian state media reported.
In neighbouring Syria, 14 people were wounded "after pagers used by Hezbollah exploded", said a Britain-based war monitor, the Syrian Observatory for Human Rights.
Hezbollah blamed Israel for the blasts and warned it would be punished.
The United States, Israel's top arms provider and close ally, was "not involved" and "not aware of this incident in advance", said State Department spokesman Matthew Miller.
The afternoon blasts hit Hezbollah strongholds across Lebanon and dealt a heavy blow to the militant group, which already had concerns about the security of its communications after losing several key commanders to targeted air strikes in recent months.
Hezbollah had instructed its members to avoid mobile phones after the Gaza war began and to rely instead on the group's own telecommunications system to prevent Israeli breaches.
"Hundreds of Hezbollah members were injured by the simultaneous explosion of their pagers" in the group's strongholds in Beirut's southern suburbs, in south Lebanon and in the eastern Bekaa Valley, a Hezbollah source said.
Since October, the unabating exchanges of fire between Israeli troops and Hamas ally Hezbollah in Lebanon have forced tens of thousands of people on both sides of the border to flee their homes.
Not formally declared as a war by Israel, the exchanges of fire between Israeli troops and Hezbollah have killed hundreds of mostly fighters in Lebanon, and dozens on the Israeli side.
On Monday, Israeli Defence Minister Yoav Gallant warned that failing a political solution, "military action" would be "the only way left to ensure the return of Israel's northern communities".
Before the wave of pager explosions, Israel said it killed three Hezbollah members in a strike on Lebanon.
'Israel sabotaged the pagers before delivery'
Israel apparently infiltrated a supply chain to cause the simultaneous explosion of hundreds of Hezbollah pagers, analysts say. Israel had corrupted the devices before delivery, allowing them to explode at a specific time, they said. "The pagers that exploded concern a shipment recently imported by Hezbollah of 1,000 devices, which appear to have been sabotaged at source. One of the factories Israel owns manufactured and shipped these explosive devices that exploded today, " analysts said.
