Londoners on income support will be required to pay only half the fare on buses, Mayor Ken Livingstone has said.
Mayor says bus fares will see a 10% drop in September
At least 250,000 people, including single parents, carers and disabled people, will benefit from the low fares costing 50p for a single journey.
The mayor said the low fares follow an oil deal with Venezuela which helped slash fuel expenses for buses by 20%.
Fares will fall to 45p at the end of September after a 10% across-the-board reduction in prices of bus tickets.
Mr Livingstone said: "From today, all Londoners on income support are eligible for half price travel on London buses.
"This will make it cheaper and easier for people to go about their lives and get the most out of London.
"The agreement which makes this possible will also benefit the people of Venezuela, by providing expertise in areas of city management in which London is a world leader."
But London Assembly Conservative leader Angie Bray said Mr Livingstone should have appealed to the Treasury if he needed financial support.
"Most Londoners will reflect that the mayor of one of the richest cities in the world buying popularity off the backs of those in one of the poorest cities in the world beggars belief," she said.
"The spectacle of our mayor, who supposedly believes in social justice, going cap in hand to a dictator with a monstrous human rights record - and who presides over a sizeable portion of people in the direst poverty - to skim off a resource which is needed to relieve such poverty is morally indefensible."