Shocking footage shows youths in Brussels yelling 'Allahu Akbar' as they set fire to a Christmas tree in a public square using a petrol bomb.

Bystanders are seen looking on as the group use what appears to be a home made bomb to set the tree ablaze.

In the seconds following the loud bang, the flames engulf the tree as the group run away yelling 'Allahu akbar' - Arabic for 'God is great'.

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The youths gather around the tree (left) before a loud band is heard and it bursts into flames (right)

The group of teenagers then run away from the scene while shouting 'Allahu akbar' - Arabic for 'God is great'

After the video was uploaded online, many took to Facebook to express concern. According to The Daily Star, one wrote they were 'shocked' by what they saw in the city.

The person wrote: 'Anyone who has ever had the opportunity to travel abroad may realise that the situation in Brussels is neither normal nor tenable in the long-term.'

A second person called it an 'act of racism', and an attack on the country's customs.

Only yesterday did Belgian authorities scale down heightened security adopted over fears of a militant attack in the capital on New Year's Eve.

Belgium's crisis centre, a state body that advises the government on security, said the alert status for potential targets such as the police, military and Brussels's main square was downgraded to level two.

However, the overall security level for the country, which has been at the heart of investigations into attacks in Paris on November 13 in which 130 people were killed, remains at level three, one notch below the maximum alert status.

Brussels had called off the city's traditional New Year's Eve fireworks over an alleged plot to attack the capital.

Two of the Paris suicide bombers, Brahim Abdeslam and Bilal Hadfi, had been living in Belgium, and at least one man was suspected of having coordinated the attacks by mobile phone from Belgium as they were being carried out.