Hundreds of people have been killed in fighting between Christian and Muslim communities vying for economic and political dominance in the fertile central state.
Obasanjo said the killings, which culminated in a massacre of Muslims in May and sparked reprisal riots against Christians in the northern city of Kano, could have engulfed the whole country in chaos.
After declaring the state of emergency on 18 May, Obasanjo asked the national assembly in the capital Abuja to approve additional regulations, giving his administration powers to detain suspects, call curfews and stop public meetings.
"The eight regulations were approved yesterday," senate spokesman Henry Ugbolue said on Wednesday.
Obasanjo suspended Plateau's state governor earlier this month, calling him "weak and incompetent", and replaced him with a retired military general, Chris Alli.
Obasanjo has warned other governors they will be held responsible for any further crises in their respective states.
Civil-rights leaders say Obasanjo has overstepped his constitutional powers by requesting the increased powers and suspending the governor.
"We have launched our case in the Federal High Court in Abuja to sue, among others, the president, the National Assembly and Chris Alli," said Gani Fawehinmi, a leading human-rights lawyer and activist.
Already, the governor of the oil-rich state of Delta has launched joint military operations to quell tribal violence, piracy and criminal gangs.
Rival ethnic groups signed a peace accord there on Tuesday, although it was not clear whether it would herald a subsidence of violence in the remote region of swamps and river channels.
Over 10,000 people have been killed in religious and sectarian violence in Nigeria - Africa's most populous country - since Obasanjo's election in 1999, which marked the end of 15 years of military rule.