Iran's rulers showed they were fearing another bout of mass unrest on Sunday when they deployed riot police across the capital at the same time as a biting austerity programme took effect.
International sanctions have forced Tehran to quadruple petrol prices overnight in the first phase of a subsidy-cutting exercise that the US hopes will severely weaken Mahmoud Ahmadinejad, the Iranian president.
Sanctions are substantially hampering the country's ability to profit from its vast energy reserves, and Mr Ahmadinejad has been forced to conclude that his government can no longer afford subsidies on fuel and food that amount to Â£64 billion a year.
The deployment of riot police at the same time reflects the view that as ordinary Iranians contemplate the consequences of their government's international isolation, the government may have to pay a heavy political price.
Recent history suggests that the Iranian response is unlikely to be a shrug of the shoulders. When the government imposed limited rationing on subsidised fuel in 2007, Iranian protestors responded by setting alight dozens of petrol stations in Tehran.
Mr Ahmadinejad has appealed for understanding. "The people's cooperation and frugality are essential," he said in an interview on Iranian state television. "The successful implementation of this plan will benefit the whole nation."