The locking out of biometrically unregistered phone users' SIM cards will be done in phases.
Other 318, 950 phone users risked to be locked out in the second phase after they had failed to register their SIM cards biometrically.
The registration of SIM cards is mandatory under the Electronics and Postal Communications Act of 2010.
At least 656,091 mobile phone users in Tanzania were locked out on Monday night after they had failed to register their SIM cards biometrically by the January 20 deadline, an official said on Tuesday.
James Kilaba, the Director General of the Tanzania Communications Regulatory Authority (TCRA), told state-run Tanzania Broadcasting Corporation that the locking out of biometrically unregistered phone users' SIM cards will be done in phases.
Kilaba said other 318, 950 phone users risked to be locked out in the second phase after they had failed to register their SIM cards biometrically.
He said: "We are switching off SIM cards which haven't been biometrically registered in phases due to technicalities. You cannot just switch off 10 million subscribers at once."
According to the latest figures by TCRA, about 48.8 million SIM cards are in use throughout the country by mobile telephony subscribers.
However, only about 28.4 million SIM cards had been registered under the biometric system by January 19.
On December 27, last year, Tanzanian President John Magufuli extended the deadline for biometric registration of phone users' SIM cards for 20 more days beyond the earlier deadline of December 31 set by the nation's communication watchdog.
Speaking shortly after he had biometrically registered his SIM card in his native town of Chato in the northwest region of Geita, Magufuli said the extension was intended to give chance to people who failed to register on various reasons.
A statement by the Directorate of Presidential Communication at State House said the president insisted that there should not be further extension after January 20.
The registration move was aimed at getting rid of dishonest telephone line users, controlling cybercrime and protecting Tanzanians against those abusing the telecom sector.
The registration of SIM cards is mandatory under the Electronics and Postal Communications Act of 2010, and under this law it is an offense to sell or distribute an unregistered SIM card, to use an unregistered SIM card, to fail to record sale of an SIM card and to provide false information or statement during registration.