The BBC hopes Persian TV will reach 10 million people a week by 2012
The BBC's newest satellite TV channel has gone on air, a Farsi language service for viewers mainly in Iran, Afghanistan and Tajikistan.
BBC Persian TV will broadcast for eight hours a day, seven days a week, in peak viewing time in those countries.
The head of BBC World Service, Nigel Chapman, says millions of Iranians have dishes and there is plenty of demand.
But Iranian officials have branded the channel a "security threat" and say they will take "necessary" steps.
Iranian Intelligence Minister Gholamhoseyn Mohseni-Ezhei said after Wednesday's cabinet session: "We do not consider this network as suitable for the security of the country and will take the necessary measures with regards to it."
The Iranian authorities have already refused the BBC permission to conduct any production operations for the new TV channel on their soil.
The BBC has been broadcasting in Farsi on the radio since 1940 and it launched BBCPersian.com in 2001.
The radio has a weekly audience of 10 million people and the BBC hopes that the television service will reach the same figure within three years.
As part of the BBC World Service, the new channel is funded by the UK Foreign Office at an annual cost of £15m ($22m) but it is editorially independent of the government.
Mr Chapman said it was important to provide TV in Farsi because of the numbers of people accessing news and information through television. He called them "a very important audience... who trust the BBC, who value it".
As well as news, the BBC Persian channel will show BBC arts, culture, science and technology programmes, dubbed into Persian.
Correspondents say despite the efforts of the government in Tehran, the BBC has a high reputation as a reliable news provider among Iranians.
Although the government has banned Farsi-language operations, it does allow the BBC to base an English-language correspondent in the country.
But the authorities have warned citizens not to get involved with the new TV channel, and the BBC has advised viewers not to risk getting themselves into trouble in order to take part in phone-ins and other interactive broadcasts.