* Arsal fighting sparked by arrest of Nusra commander
* At least 50 rebels killed in ambush across border in Syria
* Lebanon buffeted by regional sectarian tensions (Adds air strikes, army statement, background)
By Alexander Dziadosz
BEIRUT, Aug 2 (Reuters) - Islamist gunmen seized a police station in a Lebanese border town and killed two soldiers on Saturday, drawing a warning of a "decisive" response from Lebanon's army to the most serious spillover from the Syrian civil war in months.
Two Lebanese civilians were also killed when they tried to prevent fighters from the Nusra Front, al Qaeda's branch in Syria, from storming the security building in the town of Arsal, security sources and state media said.
In other violence on Saturday, just across the border in Syria's Qalamoun region, the forces of President Bashar al-Assad killed at least 50 rebels, including from the Nusra Front.
Lebanon, a country of about 4 million still recovering from its own 1975-90 civil war, has been badly buffeted by regional sectarian conflicts that have seen the rise of hardline Islamist factions from Iraq to the Mediterranean.
It has been hit especially hard by the more than three-year-old war in its larger neighbour Syria, where Sunni Muslim rebels are fighting to overthrow Assad, a member of the Shi'ite-derived Alawite minority.
Arsal, a Sunni town where tens of thousands of Syrian refugees have fled to escape fighting across the mountainous border, has seen frequent spillover from Syria and has been a tinder box for domestic tensions in Lebanon.
Syrian warplanes have bombed the town's environs and rebel fighters often cross the porous frontier, sometimes resting or seeking medical treatment in Arsal.
Tensions in the area have been further stoked by the entry of Shi'ite militant and political movement Hezbollah into Syria's fray to aid Assad.
On Saturday the group took part with Assad government forces in the ambush just over the border in Syria's Qalamoun region.
Saturday's violence in Arsal began after Lebanese authorities arrested a Nusra Front leader, Emad Jumaa, at a checkpoint near town, security sources said. In response, masked gunmen fanned out in the area and stormed the police station.
One Nusra Front fighter told Reuters the fighters would not leave the station until Jumaa was released.
ARMY VOWS "FIRM RESPONSE"
The Lebanese army said in a statement it "would not allow any party to transfer the battle from Syria to its land".
"Everyone today is called upon to be aware of the seriousness of what is happening and what is in store for Lebanon, and the Lebanese, and the army," it said.
"It appears the armed operation was not spontaneous, but planned and studied, and the army will be firm and decisive in its response."
Images on local television showed military vehicles loaded with soldiers in fatigues rolling down the road in the area of Arsal as ambulances with sirens blaring drove in the other direction.
Witnesses said Syrian warplanes HAD bombarded rebel positions on the town's outskirts.
Earlier in the day, pro-Assad forces backed by Hezbollah fighters killed "dozens" of rebels including from the Nusra Front and the Islamic State, an al Qaeda offshoot, near the Syrian town of al-Jobeh, around 10 km (6 miles) from the border, the British-based Syrian Observatory for Human Rights said.
Pro-Assad forces have largely pushed insurgents from towns and villages in the border region since March, but some pockets have held out.
The Observatory's director Rami Abdurrahman said it was not clear exactly how many rebels had died in the fighting, but the total was "at least 50".
Lebanese and Syrian security sources put the death toll much higher, saying around 170 rebels were killed, but this could not be independently confirmed. They said nine Syrian government fighters and two Hezbollah fighters had also died.
Over 170,000 people have died in Syria's three-year conflict, the Observatory says. (Editing by Gareth Jones)