The outcry over a ban imposed on anti-war protesters planning to march during president Bush's trip to London over the weekend is growing.
Liberal Democrat home affairs spokesman Chris Huhne has sent a letter to the home secretary urging her to review the decision.
The demonstrators have been offered alternative routes but are banned from walking down Whitehall.
"A march down Whitehall is important to us because we want to hand our petition to Downing Street and now we can't get there," a Stop the War spokesman told politics.co.uk.
In his letter Mr Huhne, whose party was the only one to oppose the war, wrote: "As you will be aware the Stop the War Coalition have organised dozens of peaceful marches past Downing Street, and I am deeply concerned that the request has been denied.
"In this country we have a long tradition of peaceful protest and I would be shocked if British civil liberties were curtailed at the request of a foreign government. I hope that you can also confirm that the decision of the Metropolitan Police was not made at the request of the US authorities."
"Just because the votes of these protesters cannot be bought does not mean that their voices should not be heard by those in 10 Downing Street."
The letter comes a day after shadow home secretary David Davis resigned from the shadow cabinet and as an MP to challenge the erosion of civil liberties in Britain.
So far there has been no comment from the Home Office.
The Home Office stressed the march had not been banned, but that protestors simply had to take a different route.
A spokesperson told politics.co.uk: "The government fully supports the right to peaceful protest. The operational responsibility for policing demonstrations in London rests with the metropolitan police."