Venezuelan President Hugo Chavez has warned the US that any attack on Iran will have devastating consequences and send oil prices soaring.
Jubilant supporters greeted Mr Chavez in London
Mr Chavez, on a two-day trip to the UK, called for a socialist new world order and said nations were cowards for not standing up to the "American empire".
The US has not ruled out military action against Iran over its nuclear programme but is pursuing diplomacy.
Hundreds of supporters gave Mr Chavez a rapturous welcome in north London.
The president told the meeting in Camden, hosted by Mayor of London Ken Livingstone, that a US military attack on Iran would lead to Tehran cutting off its oil supply.
"If the United States attacks Iran... oil could reach $100 (£52) a barrel or more," he said.
"Moreover, Iran has said it would attack Israel, and I know they have the wherewithal to do so.
"This would be a terrible escalation and I do not know where it would end and I do not know who would get out the first nuclear bomb or how many people would die. No-one would be safe from this madness."
Mr Chavez said the US "doesn't know what to do" in Iraq, which he called "the Vietnam of the 21st Century".
Jubilant supporters cheered him, many clad in Venezuela's red, blue and yellow and banging drums, while hundreds more were left outside the packed ballroom.
Mr Chavez said capitalism was a "destructive" system and that "socialism is the way forward".
Mr Chavez called unrestricted free trade "a trap by the world's most powerful so they can keep the weakest in slavery".
Britain and a number of other countries have criticised Mr Chavez's moves to exert greater control over his country's oil reserves - the world's fifth largest.
On Friday Prime Minister Tony Blair urged Venezuela to use its energy resources responsibly.
The president's trip has been described as a private one - he has made no request to see Mr Blair or any government officials.
On Monday Mr Chavez will have lunch with Mr Livingstone, and will deliver a lecture in the evening.
BBC Americas analyst James Painter says Mr Chavez's strategy for this trip is similar to the one he used when visiting the US in September last year. He does not meet government officials but rather appeals over their heads to appear as a man of the people.
He says Mr Chavez does not want a "European flank" opening that would support the critical comments made of his government by the US.