The previous march attracted more campaigners
Metropolitan Police, who arrested two people for possession of cannabis during the march, said the event was peaceful.
Organisers were concerned before the event that thousands of campaigners would stay away because of the fear of terror attacks on the city.
The marchers, mostly from the 60-group Cannabis Coalition, walked from Hyde Park to Trafalgar Square.
Cannabis cafes
The Coalition supports plans to open Dutch-style cannabis cafes in Britain with the first proposed for Brixton, south London.
In an experiment expected to last six months, police in Lambeth are not arresting anyone found in possession of small amounts of cannabis.
New Scotland Yard said the two people arrested on Saturday may be released with a warning, but did not rule out a court appearance at a later date.
Chris Sanders, a coordinator for the Cannabis Coalition based in Kings Cross, said around 15,000 people attended the last march in Brixton in June.
'Extend experiment'
One of the scheduled speakers on Saturday was Colin Davies, who was arrested after attempting to open the UK's first cannabis cafe in Stockport, Greater Manchester on 15 September.
Mr Sanders said: "We want the experiment in Brixton to be extended throughout the UK.
"Cannabis cafes would supply the drug for personal use and for those people who need it medically.
"We have a lot of support amongst the medical profession, but the drug companies are never going to agree to something that would cut millions off their profits."