At least one petrol station has been set on fire in the Iranian capital, Tehran, after the government announced fuel rationing for private motorists.
Iranians were given only two hours' notice of the move that limits private drivers to 100 litres of fuel a month.
Despite its huge energy reserves Iran lacks refining capacity, forcing it to import about 40% of its petrol.
Tehran is trying to rein in fuel consumption over fears of possible UN sanctions over its nuclear program.
Iran fears the West could sanction its petrol imports and cripple its economy.
'Dangerous move'
The restrictions began at midnight local time on Wednesday (2030 GMT Tuesday).
The BBC's Frances Harrison in Tehran says there is anger and frustration the government did not give people more notice.
"Guns, fireworks, tanks, [President] Ahmadinejad should be killed," chanted angry youths, throwing stones at police.
Eyewitnesses have seen at least one petrol station in the outskirts of the west of Tehran on fire.
All over the city there are huge queues and reports of scuffles at petrol stations as motorists try to beat the start of the rationing and fill their tanks.
"I think rationing is not bad by itself but it must be organized," one man told the Associated Press news agency.
"One cannot announce at 9pm that the rationing would start at midnight, they should have announced the exact date at least two days earlier."
Iran's petrol is heavily subsidized, sold at about a fifth of its real cost.
The price of 1,000 rials ($0.11) per litre makes Iran one of the cheapest countries in the world for motorists.
So far there has been no announcement about whether Iranians can buy more petrol at the real market cost.
Licensed taxi drivers will be able to buy 800 litres a month at the subsidized price.
US pressure
Our correspondent says rationing fuel is only likely to add to high inflation.
It is a dangerous move for any elected government, especially in an oil-rich country like Iran where people think cheap fuel is their birthright and public transport is very limited, she says.
The US, which is leading efforts to pressure Iran to suspend its uranium enrichment activities, has said Iran's fuel imports are a point of "leverage".
Washington and other Western nations accuse the Islamic Republic of seeking to build nuclear weapons.
Iran says its nuclear program is entirely peaceful and is solely aimed at producing civilian nuclear power.
PHOTO CAPTION
Iranians set fire to a petrol station in Tehran