Videos posted on social media show large crowds of protesters marching through Tehran and other Iranian cities in what is said to be the largest show of force by the opponents of the clerical establishment in years.

Rallies in Tehran and Iran's second city, Mashhad, can be seen in footage verified by BBC Persian, with peaceful demonstrations not dispersed by security forces. 

On Wednesday, there were violent clashes in a number of cities.

The crowds in Mashhad called for the overthrow of the Supreme Leader, Ayatollah Ali Khamenei, and the return of Reza Pahlavi, the exiled son of the late former shah, who had urged his supporters to take to the streets.

It is the 12th straight day of unrest in Iran which has seen protests spread to 140 cities and towns across all 31 provinces, according to the US-based Human Rights Activist News Agency (HRANA).

It has reported that at least 34 protesters and seven security personnel have been killed during the unrest, and that 2,270 protesters have been arrested.

Another group, Norway-based Iran Human Rights has said at least 45 protesters, including eight children, have been killed by security forces.

BBC Persian has confirmed the deaths and identities of 21 people, while Iranian authorities have reported the deaths of five security personnel.

The protests began on 28 December, when shopkeepers took to the streets of the capital, Tehran, to express their anger at another sharp fall in the value of the Iranian currency, the rial, against the US dollar on the open market.

The rial has sunk to a record low over the past year and inflation has soared to 40% as sanctions over Iran's nuclear programme squeeze an economy also weakened by government mismanagement and corruption.

University students soon joined the protests and they began spreading to other cities, with crowds frequently heard chanting slogans against the clerical establishment.