One website is calling on protesters to gather at Berkshire House
Central London is preparing for thousands of protesters to take to the streets on May Day.
There are concerns the traditional workers' day of celebration will be marred by outbursts from groups intent on violence.
Protesters are expected to demonstrate on issues ranging from alleged corporate greed to the war in Iraq.
Oil and arms companies, firms with connections to Iraq and government buildings are all being targeted by protesters, with the event going under the banner "Mayday 2003 Weapons of Mass Construction - Our Day".
Up to 60 firms are on an internet so-called "Hit List" for protesters, according to one report.
Berkshire House in High Holborn began boarding up on Wednesday after it was named on one website as the meeting place for protesters demonstrating against arms company Lockheed Martin.
May Day map of London
Events, locations and times
In the Strand, where the headquarters of Shell UK may be a target, some businesses also began to board up their windows.
But local workers did not seem particularly worried.
Office worker, Julie Unite from east London told BBC News Online: "I'm just hoping it doesn't affect me. I'm making no special arrangements."
Robert Jeffries from Harrow said: "I'm aware something may be happening but hopefully it won't affect me. I've got to come to work but no special arrangements have been made."
And smaller businesses, such as tourist kiosks in Trafalgar Square and Oxford Street are unlikely to open on Thursday.
In Oxford Street, stallholder Mark Twomey, said he would not be opening.
"But it may not affect this area much", he said. "I've heard it's supposed to be in the Holborn area this year."
The Strand may be a target for protesters
One group of American tourists had not heard that demonstrations were planned.
"We've been here three weeks and weren't aware of anything happening," Tom Wallace, from Florida, said.
"We're going home tomorrow but we'll make sure we keep away from the centre."
The Metropolitan, City of London and British Transport Police forces have spent months preparing everything from road closures to riot control if necessary.
About 4,000 officers are expected to be on duty and reserve units in the different London boroughs will be on standby in case of major trouble.
But not everyone is expecting the worst.
Guy Taylor, from Globalise Resistance, said: "We have seen some of the biggest demonstrations in British history recently, especially against the war, with more than one million people on the streets and no violence.
"I don't see why this should be any different."
The Met's deputy assistant commissioner Andy Trotter, responsible for the May Day police operations, said his men were happy for people to demonstrate lawfully but warned they would "deal firmly" with anyone who broke the law.
Thousands of trade unionists are expected to take part in the annual TUC march from Clerkenwell Green.
They will be joined by a variety of other demonstrations, including the usual Critical Mass bike ride, students fighting tuition fees, a protest against the use of prison labour and anti-fur campaigners.
Stop the War has also been involved and thousands of first-time May Day protesters are also expected.
May Day events also include demonstrations in Liverpool, Glasgow, Edinburgh, Manchester, Leicester and Bradford.