Ken Livingstone has been criticised for a proposed deal to get cheap Venezuelan oil for London's buses, in return for consultancy services.
President Chavez was a guest of the mayor in May
The London mayor confirmed his officials were in discussion with Venezuelan president Hugo Chavez.
London would offer help on issues such as transport and the environment.
But the Tories questioned the impact it would have on Venezuela's poor, while the Greens said he should be reducing the city's dependence on fossil fuels.
There have been concerns that Tube fare rises in central London for 2007, announced this week, will hit the lowest income groups hardest.
The deal could subsidise Oyster travel cards for the poorest Londoners, Mr Livingstone said.
Mr Livingstone told the London Assembly on Wednesday: "I suspect we are two or three months away from finalising this deal, but it was certainly the case that President Chavez wanted to target the benefit to the very poorest, the unemployed."
Venezuela has the largest oil reserves in the Americas, and its socialist president is the Bush administration's fiercest critic in Latin America.
But in November 2005, it signed a deal with the US state of Massachusetts to provide cheap heating oil to poor households. Similar deals were also signed in Boston and New York.
President Chavez visited London as Mr Livingstone's guest in May.
Proposals for the London deal include offering the city's experts in environmental and transport issues as consultants to the civic authority in the Venezuelan capital, Caracas.
But news of the discussions has angered some of Mr Livingstone's political rivals.
Angie Bray, Conservative leader on the London Assembly, said: "Ken Livingstone and the President of Venezuela should be ashamed of themselves for even contemplating such a proposal.
"I'm sure the 35% of Venezuelans who struggle below the poverty line, many of them critically so, would be shocked at the cynical siphoning off of their main asset to provide one of the world's most prosperous cities with cheap oil. "
Meanwhile Darren Johnson, of the Assembly's Greens, said the deal sent out the wrong message.
"London needs to be working on reducing its dependence on fossil fuels, not finding ever-cheaper sources of it," he said.
"If Ken is really interested in the environment and international co-operation then he should be working on exporting clean energy expertise to Caracas."