British Prime Minister Gordon Brown urged the public on Saturday night to remain vigilant and support the police after the national security alert level was raised to "critical."

He reassured people that it is right to raise security level at airports and crowded places, and safety of the people is the government's first duty.

"The British people must remain united, resolute and strong," he said after chairing a Cobra meeting with the Home Secretary and the security officers.

The highest security alert level means imminent attack is expected.

The Home Office has also ordered security to be tightened across the country.

A burning car was driven at the front of the main terminal building at Glasgow Airport, at around 3:15 p.m. on Saturday afternoon. Two people were arrested and the airport was evacuated and closed following the incident.

Witnesses described the car as a Jeep Cherokee which was driven at speed towards the building with flames coming out from underneath.

Shortly after the incident, vehicle access were closed at New Castle and Edinburgh airports on heightened state of alert, while armed police closed off Blackpool airport at the west coast of Britain.

The incident is now treated as a terrorist attack. And police are calling on people to be vigilant.

Related:

Burning car crashes into Glasgow Airport

LONDON, June 30 (Xinhua) -- A burning car was driven at the main terminal building at Glasgow Airport, northern Britain on Saturday afternoon.

Two people were arrested and the airport was evacuated and closed following the incident, Sky news reported.

Witnesses described the car as a Jeep Cherokee which was driven at speed towards the building with flames coming out from underneath.

The incident is now treated as a terrorist attack. And police are calling on people to be vigilant.