Three of the seven people arrested in Brighton in connection with botched bomb attacks on London have been released.

The two men and one woman were freed without charge yesterday from an undisclosed police custody centre in Sussex where they had been held since Sunday.

The four other men arrested at a block of flats in Dyke Road, Brighton, were still being questioned.

Metropolitan police yesterday were granted detention orders to hold the four until Friday.

The first-floor flat at Fairways Flats, meanwhile, was still under police guard last night while scenes of crime officers continued searching the property.

Police kept observation on the flats from the nearby private Windlesham School prior to the raid.

Police yesterday were also guarding a semi-detached house in Southall Avenue, Moulsecoomb, which was also raided on Sunday.

No arrests were made at the house but forensic experts yesterday were combing the rooms in connection with the July 21 failed suicide bomb attacks in London.

Students living at the property were ordered out on Sunday and have not been allowed to return, even to pick up personal belongings.

The July 7 suicide bombings and July 21 attempted bombings have resulted in a heightened state of alert in Brighton and Hove, which is preparing to host next month's Labour Party conference.

Security for the conference will cost at least £1 million more than in previous years.

More than 1,000 police, some of them armed, will be at key points for the event at the Brighton Centre between September 25 and 29.

Police are working closely with the military and security services for Operation Otter, which last year cost £2.4 million but this year is expected to cost a record £3.5 million.

Police and organisers are already urging the public to be vigilant although they stress they have no indication of any specific threat.

A joint council and police letter to 1000 residents and businesses near the venue says the eyes and ears of the public were the most effective weapon against terrorism.

The letter, sent this week, reads: "Any suspicious activity should be reported to the anti-terrorist hotline 0800 789321 or Sussex Police on 0845 6070999. In an emergency, always call 999.

"Things to look out for include suspicious persons looking to rent property or buy vehicles, attempted credit card frauds, or suspect bags and packages.

"Residents are advised to trust their instincts and alert police if they feel something is wrong."

A secure "Island Site" will be created around the Brighton Centre, Grand Hotel, Metropole Hotel and Russell Road car park.

This will be in place from Friday, September 23, at 8pm until the afternoon of Thursday, September 29.

Residents and businesses will need vehicle passes to enter the island site. Application forms are available from www.sussex.police.uk or Operation Otter, PO Box 424, Brighton BN1 2YL.

Streets affected are Russell Square, Clarence Square, Cannon Place, St Margaret's Place, Russell Mews and Russell Place.

Council head of tourism Adam Bates said: "The conference itself earns city businesses about £10 million. It also helps attract other very lucrative conferences for them which cause no disruption at all."

Chief Inspector Lawrence Hobbs of Sussex Police said: "We'll be doing everything we can to keep everyone safe and maintain business as usual for the city."

The newsletter to residents and businesses in the conference area states: "Planning for Operation Otter is well under way. We have planned for a very high threat level and will be thoroughly testing these plans and where necessary may increase the security even further."

The letter tells how further security advice can be obtained on the Home office web site www.homeoffice.gov.uk/terrorism and www.mi5.gov.uk