"I intend to go all the way", Liviu Dragnea told reporters after a closed doors meeting with leading members of his left-wing Social Democrat Party (PSD).
"We must not hesitate in finishing the fight to normalise the justice laws (and) penal codes," he said.
Romania's high court on Thursday sentenced Dragnea, widely acknowledged as the most powerful politician in the country, to three-and-a-half years in prison over a fake jobs scandal.
The Court's decision can be appealed, but comes as a blow to his political ambitions because it is his second conviction.
Dragnea has been unable to become premier because of an earlier two-year suspended prison sentence for vote-rigging dating back to 2016.
He is also under investigation for fabricating documents to obtain EU funds in a third case.
"We saw a mass execution. They sentenced nine people just to justify my verdict", said Dragnea referring to those who were handed jail terms alongside him on Thursday.
Dragnea, who also presides over the lower chamber of Parliament, added that he was unwilling to resign from that position either.
He said that from now on his party would act in a way that was "firm and radical".
The warning stoked fears among the opposition that the social democrats might amend some laws to benefit Dragnea in coming weeks and might try to impeach centre-right President Klaus Iohannis who often clashes with the government.
Prime Minister Viorica Dancila, hand-picked by Dragnea for the position, raised eyebrows earlier on Friday after saying that "the decision of the Court shows that in Romania the law is still applied arbitrarily".
Other PSD members labeled the verdict politically biased and blamed the "parallel state", often invoked by the ruling coalition when it's leaders are mired in legal problems or when trying to fire antigraft prosecutor Laura Codruta Kovesi, one of the party's main targets.
The Supreme Court, which handed Dragnea the jail term, reacted by criticising politicians and stressing that "justice is made only in courtrooms".
Judges asked the body that oversees the judicial system to publicly defend their reputation.
"(...) These kind of statements may have a negative impact on the judiciary, affecting it's independence, prestige and credibility", said the Judges' Forum Association in a statement.
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