Police have pledged to keep Brighton and Hove open for business today as an alliance of protest groups takes to the streets.

Senior officers are ready to divert traffic and public transport in case roads are blocked but say they will not flood city centre streets with uniformed officers ahead of the event.

Organisers of the May Day “street party and protest” have defended their decision not to discuss their plans with police, despite public appeals for them to help minimise disruption.

Previous demonstrations by the same groups have seen big-name shops, banks and restaurants become targets for vandalism or invasion.

Businesses have expressed concerns visitors could stay away to avoid today's event..

Chief Superintendent Graham Bartlett said: “It is going to be busy in town but it is still open for business and we would encourage people to come into Brighton and enjoy everything it has to offer.

“We have told businesses there is a demonstration. Obviously they need to consider their own vulnerability and security.

“If there is anything that happens at their premises which is violent or damaging we will be there to support them.”

He said traffic will be available to divert traffic away from any blocked roads.

Police and activists alike will not learn where midday's May Day demonstration will begin until a last-minute announcement is made on social networking site Twitter and a phone-line.

Roger French, managing director of Brighton and Hove Bus and Coach Company, said: “It is going to be 'suck it and see' as the day goes on, which is a bit unfortunate because we can't plan for what might develop.

“If they block the Clock Tower, it brings the whole city's bus network to a halt.

“If it does break into the city centre it will inevitably disrupt thousands of people.”

David Sewell, of the North Laine Traders' Association, said the threat of disruption could put people off coming to the city.

He said: “It is the opposite of a good weather forecast.“People think, 'If we go to Brighton, let's not get mixed up in that, let's go another day'.”

The coalition, or alliance, of 12 different fringe groups has pledged to create a “carnival atmosphere” on the streets of Brighton and Hove at a May Day event.

The groups includes Smash EDO, South Coast Climate Camp, Brighton Uncut, Brighton Anarchist Black Cross, Sussex Industrial Workers of the World, No Borders Brighton, Squatters' Network of Brighton, Brighton AntiFascists, Brighton Hunt Saboteurs, students, Queer Mutiny Brighton and Brighton Benefits Campaign.

Previous targets of the groups have included Barclays bank in North Street, Vodafone and Topshop in Western Road, BHS and Dorothy Perkins in Churchill Square, the EDO factory in Home Farm Road, Moulsecoomb, Brighton Town Hall, the empty Taj shop in St James's Street, BP garages, the UK Border Agency offices in Frederick Place, Brighton, Atos Healthcare in Upper North Street and McDonald's and NatWest branches.

A Smash EDO May Day event in 2009, using a similar “secret meeting” system, began at the Palace Pier roundabout.

Uncut groups have made headlines by staging sit-ins or gluing themselves to parts of buildings they claim do not pay enough tax.

About 60 people are expected to join the event from the South Coast Climate Camp in Lewes, which has hosted training sessions in “direct action” techniques all week.

A statement from the group said: “We need dispersed, small, mobile groups, using surprise, individuality, irregularity and unpredictability to achieve momentary tactical superiority.”

Andrew Beckett, of Smash EDO, which is opposed to the manufacture of weapons components in Brighton, said: “We have asked people to bring music and drums and to have a carnival atmosphere, but a carnival of resistance.”

When asked about businesses' fears about disruption, he said the protesters were right not negotiate with police over their plans.

He said: “We do not feel that dissent expressed with state permission is worth expressing.

“We feel that we have the right to gather and to make our point heard in our city centre without having to negotiate with police to exercise that right.

The Argus will be in the heart of the city during the protests. Click here for our liveblog coverage.