Foreign minister says Iranian military is on standby after US president said he would act if demonstrators were killed

Donald Trump has threatened to intervene in Iran if its government kills demonstrators, prompting warnings from senior Iranian officials that any American interference would cross a “red line”.

In a social media post on Friday, Trump said that if Iran were to shoot and kill protesters, the US would “come to their rescue”. He added: “We are locked and loaded, and ready to go,” without explaining what that might mean in practice.

Demonstrations against deteriorating living conditions have widened to include criticism of how Iran is governed

Alborz, a textile merchant in the central Iranian city of Isfahan, decided he could no longer sit on the sidelines. He closed his shop and took to the streets, joining merchants across Iran who shuttered their stores and students who took over their campuses to protest against declining economic conditions.

The sudden loss of purchasing power pushed Alborz and tens of thousands of other Iranians into the streets, where protests are now entering their fourth day. Students have paralysed university campuses, traders have shut down their stores and demonstrators have blocked off streets in defiance of police. Protests have spread from the capital, Tehran, to cities across Iran.