In a carnival atmosphere the demonstrators - many in wheelchairs - gave an unmistakable show of growing national support for the decriminalisation of the drug.
Although a large number openly smoked cannabis on the march, police made no arrests during the demonstration. Two people were detained for drugs- related offences later.
Rosie Boycott, editor of the IoS, which launched its campaign last September, said she was "thrilled" by the turnout.
"There was every age group, every strata of society, and it was very well-behaved. Nobody flouted the law and nobody was arrested during the march. The police were incredibly helpful and full of jokes," she said. "We want politicians to see that it isn't going to lose them votes to look at the issue."
In the biggest march of its kind for 30 years, protesters met at Reformer's Tree in Hyde Park and walked down Park Lane and Piccadilly to Trafalgar Square, where campaigners, including Labour MP Paul Flynn and veteran cannabis campaigner Howard Marks, called for the drug to be decriminalised.
Some estimates put the turnout at well over 16,000, possibly up to 25,000. Scotland Yard, more cautiously, put the figure at around 11,000.
Meanwhile, a poll on national Talk Radio revealed that two-thirds of people want cannabis legalised. The Home Secretary, Jack Straw, has repeatedly said he will not seek to change the law on cannabis.
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