Police and Brighton and Hove Bus and Coach Company have released CCTV pictures showing the start of a bus fire.

They urged youngsters on the vehicle to come forward.

A police spokeswoman said: "We would also appeal to The Argus' readers to help us identify them."

Four youths have been arrested in connection with the incident but police still wanted witnesses to contact them.

Terrified passengers fled to safety as flames tore through the No 7 double-decker in North Street on Saturday.

A group of young teenage girls and boys at the rear of the top deck where seen before the fire started.

The top deck of the bus was wrecked and police and East Sussex Fire and Rescue immediately launched an investigation.

Worried members of the public contacted emergency services at 3.30pm to report smoke and flames coming from the rear of the top deck.

The bus, stationery at a stop in North Street at the time, was evacuated and police closed the road while the blaze was brought under control.

Sue Heard, Brighton and Hove Police spokeswoman, said: "This was a very serious incident and it is extremely fortunate no one was injured.

"As you would expect on a Saturday just before Christmas the bus was very busy with shoppers.

"We have a number of witnesses who saw a group of youths playing with lighters moments before the fire took hold. Unfortunately for the arsonists the bus has very good CCTV footage of the incident.

"We are confident we will catch those who are responsible."

Hundreds of people watched as flames and smoke quickly engulfed the top deck.

The drama caused city-centre traffic chaos on one of the busiest shopping weekends of the year.

Police closed the road until the wreckage was cleared.

Roger French, bus company managing director, called it attempted murder.

He said: "This was a criminal act that could have led to serious injury or even death.

"This is not a matter of kids larking about on the bus.

"This is serious and as far as I am concerned it should be classed as attempted murder.

"That bus was busy and people could have been really badly hurt."

Police are investigating an allegation that one of a group of seven teenagers aged between 14 and 15 started the fire.

They got on the bus at Brighton Marina and sat upstairs at the back of the bus as it travelled along Eastern Road, the Old Steine and into North Street.

Anyone with information about the incident is asked to contact Sussex Police on 0845 6070999 or Crimestoppers in confidence on 0800 555111.