Năstase became the country's first head of government in the post-communist period to be sent to prison for corruption.
He told the police officers waiting outside his home for him to pack that he needed to get some books from his library. A short time later, journalists and others waiting outside heard a what appeared to be a gunshot.
Năstase, who owns a gun permit as a hunting enthusiast, had tried to shoot himself in the neck, authorities said. Television reports showed him being taken to hospital and doctors said this morning that his condition is stable and he can leave the hospital in two weeks' time.
The former premier was convicted on 30 January on charges related to illegal political fund-raising for his unsuccessful political campaign in 2004.
Năstase is on trial for other two high-level corruption cases related to his abuse of power while in office. One refers to the house he bought in Bucharest's most affluent neighbourhood at 25 times less the market price, and the other one to a suspicious money transfer of €400,000 to the bank account of a purported relative.
The national anticorruption body claims that between 2001 and 2006, Năstase's bribe and abuse of power amounted to some €1.4 million.
One of Năstase's lawyers, Ion Cazacu, said he would take the case to the European Court of Human Rights and called it "the biggest judicial error" he has ever witnessed.
The former premier had previously claimed he is the victim of "orchestrated actions, motivated by strictly political orders and no legal basis".
Romanian MEP Monica Macovei told the national news agency Mesiafax: "The final sentence is a signal that Romanian politicians are not above the law. The independence model given by the judges of the high court of justice must be followed by all courts in cases involving politicians and other dignitaries - this is the real test for the judiciary".
The EU has chastised Romania for its judicial system while while pressing the country to step up its efforts in its fight against corruption. Corruption was one of the main reasons countries such as the Netherlands has sought to keep Romania out of Schengen.
The Commission's annual Cooperation and Verification Mechanism report on the judiciary and anti-corruption reforms in Romania is going to be published again next month. Romanian diplomats have previously stated that they expect this report to be the last one, since the post-accession monitoring period is ending this year. The imprisonment of its former PM on grounds of corruption could help Romania in proving its reforms.