Tabriz Prosecutor Yusef Firoozi also told Iranian ISNA news agency that more detentions will follow soon.
The Tehran-based "Iran" newspaper on May 19 published a cartoon depicting a cockroach speaking Azeri, the language of the predominant ethnic group in the northwest.
Iran's central authorities today ordered the indefinite closure of the daily and the arrest of its editor and cartoonist, according to state radio.
A number of cabinet ministers have apologized for the cartoon. Parliamentary speaker Gholam-Ali Haddad Adel called the image an insult to all Iranians.
Demonstrators Took To Streets
Hundreds of demonstrators took to the streets of Tabriz, the region's main city, to protest the publication.
Radio Farda reported that the clashes left at least one demonstrator wounded. The station reported that security remained tight late today in Tabriz, but that no new incident was recorded.
Azeris make up an estimated one-fourth of Iran's population. They are concentrated mainly in the northwest of the country, south of the border with Azerbaijan.
(Radio Farda, with additional agency reports)
MORE: To read RFE/RL's Azerbaijani Service's coverage of this controversy in Azeri, click here.