Shells fired from war-torn Syria killed eight people, including five children, when they rained down on the Lebanese border town of Arsal on Friday, officials said.
The attack was the deadliest on the Sunni town since war erupted in neighbouring Syria nearly three years ago.
The deaths came as at least 20 rockets and shells launched from Syria hit border areas in the eastern Bekaa Valley, which has seen frequent Syria-related violence.
President Michel Sleiman called on the army to "protect" border villages, while ex-premier Saad Hariri described the attack as a "massacre."
After news of the deaths broke, violence erupted in the northern city of Tripoli, a frequent scene of Syria-related clashes and killings.
Speaking to AFP, a security said eight people died in a "cross-border shelling attack against Arsal," which the army said hit several areas over a little more than an hour.
Local official Ahmad al-Hojairi said five of the dead were children, three from one family, and that people were in a state of panic.
"We are being made to pay for the crisis in Syria. What happened today is terrifying," he said.
Arsal is a Sunni area in the Bekaa Valley that hosts thousands of refugees who have fled the violence in Syria.
The Syrian air force has been bombed it several times, most recently in December in a raid that prompted the Lebanese army to hit back with anti-aircraft fire.
Arsal's population sympathises strongly with the revolt against President Bashar al-Assad.
Once dominated by Damascus militarily and politically, Lebanon is divided over the Syrian war.
The divisions have widened ever since Lebanon's Shiite movement Hezbollah acknowledged in May that it was sending fighters into Syria to support Assad.
President Sleiman renewed his implicit warning to Hezbollah over the affect this was having on Lebanon.
Signup to our newsletter and follow us on Facebook and Twitter!
"The consequences of even deeper involvement in the Syrian crisis is making the Lebanese pay a high price," said Sleiman, who has previously called on Hezbollah to withdraw from Syria.
He also called on "security and military officials to take all relevant measures to protect Lebanese towns and villages on the border with Syria."
Caretaker prime minister Najib Mikati also called on the army to take "the necessary steps to protect Lebanese territory."
Saad Hariri, Sunni leader of the Future bloc that opposes Assad and Hezbollah, described the attack as a "massacre."
"The murderous and terrorist rockets, regardless of their source, have a clear purpose to sow discord, and intimidate and terrorise innocent citizens," he said.
And as night fell, shelling resumed on the outskirts of Arsal, a security source said, adding that there were no casualties.
The shelling comes a day after a car bomb killed three people in Hermel, a Hezbollah stronghold in the Bekaa. The attack was claimed by the previously unknown jihadist Al-Nusra Front in Lebanon as a suicide operation.
In reaction to the Arsal deaths, residents of Tripoli's restive Sunni neighbourhood of Bab al-Tebbaneh burned tyres on the street that separates them from the adjacent Alawite area of Jabal Mohsen.
Bab al-Tebbaneh's residents support Syria's rebels while those in Jabal Mohsen are from the same religious community as Assad and support him.
Gunmen from Bab al-Tebbaneh also started firing into the air, and then clashes between the two neighbourhoods broke out, a security official in Tripoli told AFP.
"Three people have been wounded in an exchange of machinegun fire and rocket fire between Bab al-Tebbaneh and Jabal Mohsen," he said.
In another Sunni neighbourhood, gunmen stopped two civilians on the streets, and asked them for their IDs.
"They thought both were Alawites, and opened fire at them," the security official said.
"One of them, an Alawite, died from his wounds in hospital, while the other, a Sunni, is being treated for his injuries," he added.
Rights groups have condemned a spike in Tripoli attacks targeting Alawites in recent months.