But the Supreme Court ruled in a tense, almost 11-hour session by 6-5 that because the 72-year-old has already lost a lower court appeal against his conviction, he must begin the sentence.
Lula, a two-term former president who leads in polls ahead of the October 7 presidential election, has not spoken publicly since the court denied a request late Wednesday to delay his prison sentence. The ruling could affect the country's stability ahead of October elections.
Chief Justice Carmen Lucia, who was sharply criticized during the session by various colleagues, cast the deciding vote after the court was tied at 5 to 5.
"There is no justice in this decision", the PT said in a statement on Thursday, "there is a combination of political and economic interests, against the country and its sovereignty, against the democratic process, against the Brazilian people".
On the right, Lula is considered the face of corruption sweeping the country's political elite.
Leftists, however, remember Lula's 2003-2010 rule as a time when Brazil used its wealth to lift tens of millions of people out of poverty.
He has a lead in opinion polls.
Journalist Michael Fox has more.
Lula was found guilty in August and sentenced to 10 years in prison for accepting bribes worth 3.7 million reais ($1 million) from engineering firm OAS, the amount of money prosecutors said OAS spent refurbishing a beach apartment for Lula in return for his help winning contracts with state-run oil company Petroleo Brasileiro.
They have until Monday to file a final, purely technical appeal with the lower court that turned down Lula's first appeal in January.
Villas Boas wrote that the army would not now depart from its constitutional role, but some retired officers have warned that the military might indeed be tempted to intervene if the Supreme Court allowed Lula to go free on appeal.
The Supreme Court's deliberations on the sensitive case took place under extraordinary social and political pressure.
Up to 20,000 people protested in São Paulo on Tuesday calling for his immediate imprisonment, while supporters also rallied in large numbers in a rival demonstration. At the same time, the court had to contend with the fact that Lula remains enormously popular with much of the population, especially in the poorer northeast.
While da Silva, known simply as "Lula" to Brazilians, has further appeals available, he could be forced under Brazilian law to begin serving his sentence, which Moro and the other judges have ordered.
In a hammer blow to Lula's hopes, the Supreme Court (STF) ruled by a slim six-to-five majority to reject his claim for habeas corpus, leaving a slim judicial review remedy available to the former president's defence lawyers.
The head of the army, General Eduardo Villas Boas, tweeted that the military shared Brazilians' "desire for the repudiation of impunity".