Thousands of anti-war protesters have taken to the streets of Britain to show their opposition to the use of military action against Iraq.
A policeman is hurt in London
More than 5,000 young people brought central London to a standstill on what campaigners have branded a "day of shame".
Banner waving groups shouted, chanted and blew whistles outside Parliament - hours before an organised demonstration by the Stop the War coalition got underway.
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.
And 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.
Most protesters remained calm but there were some involved in isolated clashes with Metropolitan Police.
Metropolitan Police Commissioner Sir John Stevens warned the protests "draw police away from their very important duties at this time in protecting vulnerable locations and those communities who may feel threatened and fearful.
"We are disappointed that today's protests included a significant number of young people playing truant from school.
I still believe we should tell people that war is wrong
"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."
Stop the War coalition chairman Andrew Murray said: "The war has started which is an outrage against
world peace, against the population of Iraq and against law and democracy in
"This is going ahead without the support of British people. This is a day of
shame for Britain. Our country has been dragged into a ridiculous war by a US
administration which has shown contempt."
Anti-war groups organising a national demonstration in London on Saturday will be boosted by the numbers that turned out into the sunshine on Thursday.
Demonstrators who gathered in Parliament Square spilled into Whitehall, forcing police to blockade the route to Downing Street with police vans.
Some protesters climbed on railings and up traffic lights to secure a better view, or to scream orders to make friends sit down, causing a bigger obstacle for officers to move.
One senior policeman used a loud-hailer to get a crowd trying to pass the vehicle blockade back into the Square, but his amplified voice was drowned out by the mêlée.
Things were slightly more orderly in the square, where young people sat on grass strewn with litter, lager cans, bikes and dogs.
Darius Zaerin, 15, and his cousin Alys, 14, donned the t-shirts and waved their colourful banner, stressing that they were representing the voices of many of their fellow pupils at Alexander Park School in Muswell Hill.
"I don't think our protest will make much difference, but I still believe we should tell people that war is wrong," said Darius.
CND said it was "appalled" by the start of the "illegal, immoral war against
"This war will, in all probability, kill and injure
hundreds of thousands of innocent people and devastate the entire nation,
weakened already by 12 years of economic sanctions," a spokesman said.
"Mr Blair has made a grave error of judgment when he abandoned the UN route to follow the US road to war."
Osama Saeed, spokesman for the Muslim Association of Britain, described Thursday's bombings as "dreadful".
If America gave Iraq back to its people, "we would come back here and apologise to America", he said, but feared the hostilities will result in "many casualties".
Labour rebel Alice Mahon told a news conference on Thursday: "This is the most one-sided war in the history of mankind."
Labour left-winger Jeremy Corbyn said: "The ordinary people of this country are showing revulsion over what is going on."