Tehran  - The cellular phone text-messaging system has been switched off in the Iranian capital, Tehran, the labour news agency ILNA reported Tuesday, after a three-week outage over and after the disputed presidential election.

The short message system, or SMS, was initially switched off June 11, one day before the presidential election, resumed again last week after 21 days but again turned off Monday.

Officials at the telecommunications ministry have so far declined to give any reason for the renewed switch-off, ILNA said.

President Mahmoud Ahmadinejad was declared the winner of the election, but his opponents charged fraud, setting off protests, in which at least 20 demonstrators are thought to have died.

Supporters of opposition leader Mir-Hossein Moussavi had called for a general strike July 6-8 throughout Iran.

But due to poor air quality in Tehran and several other cities - with pollutants reaching nine times above their normal levels - the government declared a public holiday on Tuesday and probably also on Wednesday.

The government said the pollution was caused by dust originating from western parts of the country and the deserts of Iraq and Saudi Arabia. (dpa)