Thousands of protesters opposed to the US and British presence in Iraq are taking part in marches across Europe.In London, some 20,000 demonstrators took to the streets, according to police estimates. Organisers said recent clashes in Basra involving British troops highlighted the importance of ending the UK military presence. The protest began outside the British parliament, where the fathers of two men killed in Iraq addressed the crowd. Demonstrators weaved their way through central London, before a rally in Hyde Park. Similar demonstrations are also being held today in Rome, Paris and Copenhagen.