Our prayers with the Lebanese Army and hostage.

BEIRUT (Reuters) - Islamist gunmen seized a police station in a town in Lebanon on the border with Syria and killed two Lebanese soldiers on Saturday after Lebanese security forces arrested one of their leaders, security sources and a rebel fighter said.

Two Lebanese civilians were also killed trying to prevent fighters of the Nusra Front, al Qaeda's branch in Syria, from storming the security building, Lebanese state media reported. Arsal, a Sunni Muslim town where tens of thousands of refugees have fled fighting across the mountainous border, has frequently seen spillover from Syria's more than three-year-old civil war.

One Nusra Front fighter said the fighters had seized the police station in response to the arrest of one of the group's leaders, Emad Jumaa. Lebanese authorities said in a statement he had been arrested at a checkpoint near Arsal around noon. Syria's rebels are overwhelmingly Sunni, while President Bashar al-Assad is from the Shi'ite-derived Alawite sect and has been supported by Shi'ite militias from Lebanon and Iraq. Over 170,000 people have died in the conflict. Lebanese security sources and state media said two soldiers were killed in the fighting in Arsal.

Just across the border in the mountainous Qalamoun region, at least 50 rebels were killed by pro-Assad forces backed by the Lebanese Shi'ite militia Hezbollah.

The pro-government forces had led an artillery ambush killing "dozens" of rebels, including from the Nusra Front and the Islamic State, an al Qaeda offshoot, the British-based Syrian Observatory for Human Rights said.

The ambush took place around the town of al-Jobeh, around 10 km (6 miles) from the border.

The Observatory's director Rami Abdurrahman said it was not clear exactly how many rebels died in the fighting in the region, 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.

(Reporting by Alexander Dziadosz, editing by David Evans)