Complex sentence: The largest demonstration organised by the Coalition was against the imminent invasion of Iraq on 15 February 2003, claimed to be the largest demonstration in Britain with estimates of attendance ranging between 750,000 and 2,000,000 people.
Split into simpler sentences: The largest demonstration organised by the Coalition was against the imminent invasion of Iraq on 15 February, 2003. This was claimed to be the largest demonstration ever seen in Britain with estimates of attendance ranging between 750,000 and 2,000,000 people.