Christmas shoppers fled for their lives from a packed double-decker bus which was set ablaze by teenage arsonists.

Yesterday a bus boss branded the attack "attempted murder" as police hunted for the gang.

Hundreds of people watched in horror as flames and smoke quickly engulfed the top deck of a No 7 bus in North Street, Brighton, on Saturday afternoon.

The drama caused city-centre traffic chaos on the busiest shopping weekend of the year as police closed the road while firefighters tackled the blaze.

Brighton and Hove Bus and Coach Company managing director Roger French 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."

Mr French estimated the cost of the damage at more than £10,000.

Detectives are studying CCTV footage recovered from on-board security cameras.

The fire was believed to have been started by a group of about seven teenagers aged between 14 and 15 who got on 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.

Passenger Fred Lancaster, 18, said the group of girls and boys had been smoking and were making a lot of noise.

He said: "They were shouting and screaming and you could smell that they were smoking.

"They all ran off the bus as it was coming towards the stop in North Street and, as they passed, one of them said jokingly there was a fire.

"We didn't take much notice but then we looked around and could see some flames flickering up.

"We all got off the bus pretty sharpish. There were about 20 people sitting upstairs so it was pretty busy.

"When everyone was off we turned to look at the bus and could see the flames were getting bigger. Then there was black smoke everywhere.

"We were really lucky. The bus had already pulled in at the stop so everyone was able to get out fast. It could have been a lot worse and caused a lot more problems if the bus had been forced to stop in the middle of the road.

"It was a good job the fire was noticed at an early stage or somebody could have been badly injured."

Witness Mavis Roper, 59, from Cheshire, was visiting a friend in Brighton for the weekend.

She said: "We were just enjoying doing our shopping as we went along North Street when we suddenly saw this bus in flames. There were crowds gathered around looking.

"The top bit of the bus just seemed to melt. It was really strange to see."

Student Sarah Bovingdon, from Kemp Town, Brighton, was left shaken by what she saw.

The 31-year-old said: "I was coming into West Street when I could smell smoke. I looked around and saw the bus on fire.

"It didn't seem too bad at first and there were a lot of people standing around but as I approached the smoke started getting thicker.

"I took a couple of pictures and then there was a small bang as the back window went out and everyone jumped back. The bus driver then started asking everyone to stand well away.

"The fire brigade was there really quickly and the fire was soon under control.

"It was quite nerve-wracking because at first I didn't really know what was going on or how the fire had started. When I heard the bang I immediately thought about what had happened in London in July and it left me a little shaky.

"Obviously it was nothing like that but just for a moment I found myself wondering.

"It was really crowded and there could have been a lot of damage. It was a good job everyone was able to get off so quickly before the fire spread."

Two fire crews from Preston Circus in Brighton were on the scene within minutes.

Sub officer Richard Chamberlain said: "The top back window of the bus had been blown out by the fire and we had to break the rest of the windows to ventilate it and get the smoke out.

"There is a lot of foam and fibre glass and plastic on a bus so it generates thick smoke. The top deck was pretty much destroyed."

A spokesman for East Sussex Fire and Rescue Service said: "As you can imagine, we had a lot of calls about this one as it happened in such a busy place. Our crews were able to get there very quickly to deal with it."

Mr French said: "I would appeal to anyone who may have information about what happened or know the people who did this to get in contact with the police immediately.

"I am hopeful that with the CCTV footage we will be able to help the police in tracking down the people responsible."

The wrecked bus was towed back to its depot in Conway Street, Hove, after the 3.30pm fire for forensic examination.