Farmers in Isfahan province have for years protested against the diversion of water from the Zayandeh Rud river to supply other areas, leaving their farms dry.
Thousands of protesters have converged on Isfahan in central Iran to voice their anger after the city's lifeblood river dried up due to drought and diversion.
Friday's massive protest was the biggest since demonstrations over the water crisis started on November 9.
"Thousands of people from Isfahan, farmers from the east and west of the province, have gathered in the dry Zayandeh Rud riverbed with one key demand: let the river run," a state TV journalist in Isfahan reported, broadcasting live images of the rally.
Footage aired on the channel showed men and women in a crowd spanning the riverbed clapping in unison.
"Plundered for 20 years" and "the water must return", they chanted.
Others were seen holding up banners that read "East Isfahan has become a desert" and "Our water is being held hostage", in pictures published by Iranian media outlets.
Drought is seen as one of the causes, but farmers also blame the authorities' diversion of the river water to neighbouring Yazd province.
The farming area around Isfahan was once well supplied by the Zayandeh Rud, but nearby factories have increasingly drawn on it over the years.
Government pledges to help
The Iranian government has promised to come to the aid of farmers and resolve the crisis.
"I have ordered the ministers of energy and agriculture to take immediate steps to deal with the issue," Iran's First Vice President Mohammad Mokhber said on TV.
Energy Minister Ali Akbar Mehrabian apologised to farmers for being unable to provide water for their crops.
"We hope to fill these gaps in the coming months," he said.
Iran's President Ebrahim Raisi had already met with representatives from the provinces of Isfahan, Yazd and Semnan on November 11 and promised to resolve water issues.
Drought triggers shortages
Largely arid Iran has been suffering chronic dry spells for years.
In July, deadly protests broke out in the southwestern province of Khuzestan after drought led to widespread water shortages.
Iran is one of the most water-stressed countries in the world.
Water levels in the country's lakes and reservoirs have halved since last year due to the severe drought affecting the country and the wider region, a report from Iran's space agency said in October.
The city of Isfahan is Iran's third-largest, with a population of around two million.