The Lebanese Red Cross, which gave the morning toll, said three suicide bombers also died.
In the evening attacks, at least 13 people were wounded, the state-run National News Agency said, citing hospital officials.
A military source told AFP there were four suicide bombings in the latest violence.
At least one of them struck near a church, while two bombers detonated their explosives near an armoured personnel carrier and a military intelligence unit.
A security source said two of the attacks were near the municipality building in the centre of the village.
"Clashes are ongoing on the outskirts of the village between the Lebanese army and armed groups," after the bombings, another security source said.
The mayor of Al-Qaa, Bashir Matar, issued an urgent call to residents to stay indoors in televised remarks, warning more suicide bombers could be roaming the village.
Bashir Khodr, the governor of the Baalbek-Hermel region where Al-Qaa is located, barred Syrian refugees at a makeshift camp near Al-Qaa from entering the village.
"The security situation today is above all considerations," Khodr said in a statement to LBC television channel.
Al-Qaa, one of several border posts separating Lebanon and war-torn Syria, has been on edge since dawn when residents were brutally woken up by the sound of explosions.
At least four suicide bombers hit the village from about 4:20 am (0120 GMT), the army said.
- Knocked on door -
"The first attacker knocked on one of the homes in the village, but after the resident became suspicious, he blew himself up," a military source told AFP.
Three other suicide attackers had detonated their own explosives as people began gathering to treat the wounded.
Security forces cordoned off the site of the blasts, which lies on a main road linking the Syrian town of Al-Qusayr to Lebanon's eastern Bekaa valley.
The road cuts through a residential area in the centre of Al-Qaa, and the explosions took place less than 100 meters (yards) from the village church.
George Kettaneh of the Lebanese Red Cross said there were "at least eight killed including three suicide bombers" and 15 wounded in the pre-dawn strikes.
The army said four soldiers were wounded in the attacks, which the National News Agency reported took place at 10 minute intervals.
Resident Fadi Bsherrawi said he woke up when he heard the first blast, but went back to sleep thinking it was just fighting near the border.
The second explosion drove him out of bed, he said.
Neighbours told Bsherrawi that a Muslim resident was having a morning meal before beginning his day of fasting for Ramadan when he saw a strange group of men outside.
"He went outside to talk to them and they wanted to stir things up. So his son fired on them with a hunting rifle" before the explosions started, Bsherrawi said.
Paramedics began to arrive after the first suicide attack.
"One rescue worker who was trying to carry a wounded man was killed when the second terrorist suicide bomber came," he said.
"We have guards all the time. The village is always ready and people are on edge."
- First lines of defence -
Condemnations poured after the first wave of bombings, with the powerful Shiite movement Hezbollah denouncing a "crime borne of terrorist ideology".
UN special coordinator for Lebanon Sigrid Kaag called for international support to help the Lebanese army confront "security challenges" and "the terrorist threat in Lebanon and along its borders".
Lebanese army chief General Jean Kahwaji said Al-Qaa and other border villages "represent Lebanon's first defence lines against terrorism".
Al-Qaa is one of several border posts separating Lebanon and war-torn Syria and is predominantly Christian although one district, Masharia Al-Qaa, is home to Sunni Muslims.
Suicide blasts in the area have typically targeted checkpoints or military installations and rarely include more than one attacker.
In August 2014, the army clashed with the IS and Al-Nusra Front, Al-Qaeda's affiliate in Syria, in the border town of Arsal.