A serial arsonist is thought to be behind two car fires on either side of the conference centre where Prime Minister Tony Blair was delivering a speech to union delegates.

The vehicles were set ablaze on Tuesday night in hotels either side of the Brighton Centre, just hours after the Prime Minister and Chancellor Gordon Brown had spoken to the TUC conference - prompting an evacuation of dozens of guests and delegates and a major security alert.

Police said they did not believe the attacks were connected to the TUC conference, Tony Blair's visit or to the Labour Party conference which begins in less than a fortnight.

But the incident raised questions over security for the party conference, when Mr Blair and his cabinet will be the focus of attention for the world's media, less than three months after the terrorist bombings in London.

Central Brighton police commander Chief Inspector Steve Barry said: "There is nothing to suggest these incidents were targeting the TUC conference at the Brighton Centre.

"There has been a recent spate of arson attacks across the city and we will certainly look at the possibility of those being linked to these latest incidents."

The first attack happened in The Lanes underground car park under the Thistle Hotel east of the Brighton Centre in Kings Road, Brighton at 9.02pm on Tuesday.

A window of a Renault car was smashed and an accelerant was poured on to seats and set on fire.

Smoked reached the hotel above and guests and staff were forced to evacuate.

Flames damaged a second vehicle but by 9.21pm fire fighters had swamped the area with foam. By 9.55pm, the fire was out.

Fifteen minutes later, an automatic fire alarm was triggered at the Hilton Brighton West Pier hotel, west of the Brighton Centre.

A length of scaffold was used to smash a window of a Skoda car in the hotel's underground car park before accelerant was ignited inside.

A sprinkler directly above the car put out the flames but by then the hotel had been evacuated.

Fire crews wearing breathing apparatus reported the emergency over at 10.51pm.

Police closed Kings Road to cars and pedestrians. Guests at the Thistle were taken to the nearby Queen's Hotel lounge for tea and coffee.

Many were bewildered and those who left cars in the car park waited in the cold to see when it would reopen.

Lesley Hadley, 51, of St James's Avenue, Bexhill, was due to fly to Rome yesterday but her car and suitcase were trapped in the Bartholomew Square car park, underneath the Thistle.

She stood outside in tears while police began to search the car park telling people it was unlikely to open until the morning.

Ms Hadley said: "We parked in here and went to have a coffee and when we came back a little while later it was all sealed off."

She was devastated at the time but just after midnight police allowed her to drive away.

Speaking from outside the Vatican, she said: "They bent over backwards to help me. We got to the airport with minutes to spare."

Dawn Reynolds, from Hove, had been to see The Jungle Book with her daughter at Brighton's Theatre Royal.

She said: "I came back to the car park at about 9pm and everyone was told not to go down there. I could see smoke billowing out.

"My daughter has been picked up and has gone home but I have been waiting here to see when it reopens."

The Lanes car park under Bartholomew Square and the Thistle remained closed for most of the day while repairs to the sprinkler system were carried out.

A spokesman for the Hilton said the hotel and car park were back to normal.

Meanwhile, police and East Sussex Fire and Rescue officers were continuing their investigations into the attacks and CCTV footage was being checked.

Police said a rubbish bin was set ablaze on the seafront in Hove some time after the car park attacks.

There has been a spate of rubbish bin fires across the city in recent months and police were investigating a possible link.

The two car parks will be open to the public during the Labour conference from September 25 to 30 but there will be stop-and-search checks on pedestrians and vehicles.

The £3.7 million security operation, codenamed Otter, will involve sealing off the area around the Brighton Centre, the Hilton Metropole, De Vere Grand Hotel and the Russell Road car park.

More than 1,000 police, many of them armed, will be patrolling the city in what police say will be the biggest security operation of its kind ever mounted.