Tony Blair has come under fire for failing to consult Muslim members of parliament and religious leaders on the war in Iraq.
Some 5,000 protesters were in central London on Thursday
In a letter to the prime minister, Labour peer Lord Ahmed of Rotherham also warned the conflict could lead to racial attacks on minority groups and religious institutions.
His warning comes as the Muslim Association of Britain urged Muslims to use Friday prayers to pray for peace.
Protests against the war are expected to take place across Britain throughout the day.
The Stop the War Coalition is planning to block roads in Manchester during the morning rush hour.
Demonstrators are also expected to gather outside Scottish Labour's annual conference in Dundee while protests are planned for later in the day in Caernarfon in Wales and Belfast.
Opponents of the war in Iraq are also expected to take up positions outside parliament and Downing Street.
An estimated 5,000 people brought central London to a standstill on Thursday.
Banner waving groups shouted, chanted and blew whistles outside parliament throughout the day.
Police have warned that such protests have been drawing them away from other duties at a sensitive time.
In Brighton, police had to use CS sprays after a dozen protesters forced their way into the town hall.
There were also protests in other UK cities including Glasgow, Leeds, Edinburgh, Bristol, Cambridge and Sheffield.
Police had to seal off Bristol city centre and arrest two people after about 400 protesters - some of them schoolchildren - flooded on to streets.
In Newcastle upon Tyne a crowd of up to 250 trade unionists, council workers, and university students gathered in the city centre with anti-war demonstrators, bringing traffic to a halt.
Protesters also gathered in Jersey's St Helier's Royal Square while students at schools in the West Country faced suspension for skipping lessons to attend anti-war protests. About 200 anti-war campaigners marched through Cardiff.
A Metropolitan Police spokeswoman said 17 people were arrested during the
demonstration in Parliament Square for public order offences. They included a 13-year-old boy, 15 men and one woman.
Metropolitan Police Commissioner Sir John Stevens said officers were "disappointed" that many of the protesters appeared to be children who were playing truant from school.
"We would urge parents to work in partnership with teachers and the police in dissuading young people from carrying out any further similar unorganised protests," he said.
Anti-war groups are organising what they hope will be a major national demonstration in London on Saturday.