Up to 400 poppy sellers have taken up posts at transport hubs across London to try to break the record for the most money raised in a single day.
Organisers are hoping to raise £150,000, smashing last year's London Poppy Day record collection of £60,000.
Serving, reservist and ex-Forces personnel are stationed at more than 30 Tube and mainline stations.
All collectors will then converge at Leadenhall Market in the City at the end of the day.
Fundraisers were due at their posts during the morning rush hour between 0700 GMT and 0900 GMT, over lunchtime, and during the evening rush hour from 1700 GMT until 1900 GMT.
Among the London Poppy Day fundraisers, who have been drawn from all over the world, are 300 people who work in the City of London.
Their efforts will be spurred on by The Band of the Brigade of Gurkhas playing in Leadenhall Market and military bagpipers at various Tube stations.
Benjamin Hancock, London Poppy Day co-ordinator, said: "Raising £150,000 in one day from a single city may sound ambitious, but nobody believed that we could go from £16,000 in 2008 to £60,000 last year.
"With the increased support we have for the 2010 London Poppy Day, we're determined to exceed our target this year."
The Royal British Legion said on average between £2,000 and £3,000 was raised per station on last year's London Poppy Day, with £10,000 raised at Green Park station alone.
All money raised this year will go towards the 2010 Poppy Appeal fundraising target of £36m.
This year the Legion will spend more than £72m providing assistance to more than 160,000 serving and ex-Service Armed Forces personnel and their families.
The amount raised will be announced at the end of this year's Poppy Appeal.