A government ban on using fake phones came into force at midnight, causing communication difficulties for those who owned them.
Tanzania joined Cameroon, South Africa and Nigeria in efforts to boost security and health measures by disconnecting the phones.
About 3% of mobile phones in Tanzania are fake, official figures show.
The country has about 33 million mobile phone subscribers, out of an estimated population of 49 million people.
Some 1.2 million fake phones are expected to be disconnected, local media reports.
Report says that some people have complained that they did not know they had bought fake phones.
But overall, the ban has been welcomed and many are wondering if it will also lead to a crackdown on other counterfeit goods.
The opposition Civic United Front (CUF) has said the ban “punishes” innocent Tanzanians for the government’s failure to prevent fake phones from entering the market.
Deputy Communication Minister Edwin Ngonyani said the shutdown would “strengthen efforts by the government to bring to book criminals who have been using fake handsets which cannot be traced”.
Counterfeit handsets lack authentic International Mobile Equipment Identity (IMEI) numbers that are crucial to identifying and tracking mobile phones, making it easier for criminals and militant to escape detection.
The Tanzania Communication and Regulatory Authority said the switch-off was also meant to protect users’ health.
The fear is that fake handsets, which are not subjected to safety tests, emit more radiation and contain harmful elements, such as lead.