In the parliamentary election of March 2005, he became a member of the parliament, representing the Communist Party. In the parliament, he was elected chairman of the Commission for Foreign Policy and European Integration.

Grigore Petrenko served as deputy chairman of Moldovan Parliament from 12 May to 28 August 2009. In 2014, Petrenko, alongside with Marc Tkachuk and Iurie Muntean, was excluded from the Executive Committee of the Communist Party.

At that time, a conflict arose between the leader of the Communist Party Vladimir Voronin and Grigore Petrenko, the latter accused Voronin of lie and treachery and asked him not to nominate his candidacy for president of the CPRM. Petrenko said he was ready to propose his own candidacy for party chairman. In October 2014, Petrenko was expelled from the Communist Party, charged of violating the party regulations.

In parliamentary elections of 30 November 2014, Petrenko joined the Homeland Party (Patria) that was headed by controversial businessman Renato Usatii, but two days before the elections, the party was excluded from the race.

On 10 April 2015, Petrenko joined the party Our Home – Moldova (Patria Noastra – Moldova) and was elected as party chairman. In May 2015, Petrenko joined the race for mayor of Chisinau, representing the party Our Home - Moldova and accumulated, 0.21% of votes in the elections of 14 June 2015.

Grigore Petrenko and Antifa

The press rumored that Petrenko allegedly created a very dangerous extremist movement "Antifa", which planned to organize violent anti-state actions.

According to sources, cited by Timpul.md, Antifa members were to organize a "bloody Maidan" in Chisinau. During the searches at the group members’ residences, police officers detected guns with silencers, grenade launchers, grenades, military uniforms. They also found Moldovan lei and Russian rubles. The suspects had prepared new mobile phones, SIM cards; there were access plans of many public institutions, including the Central Election Commission and state public television.

At that time, Petrenko was in Moscow and claimed that he became a victim of a media attack, which, in his opinion, "demonstrates the fear of the nationalist and oligarchic regime."

On 6 September 2015, Grigore Petrenko with a group of people under his leadership organized a protest in front of the Prosecutor General’s Office. The event turned out to be a violent one, six police officers were injured and five people were detained.

Protesters had demanded the resignation of the prosecutor general and wanted to storm the building and to install their tents in front of it.

On 15 September, Grigore Petrenko was sentenced to 30 days of detention, and then the arrest warrant was extended until 22 February, when he was placed under house arrest following the guarantees of the parliamentary faction of the Socialist Party.  Socialists demanded final release of Petrenko group members. Dodon came to Petrenko’s rescue, even if Petrenko criticized him when he left the Communist Party in late 2011.

On 26 April this year, the court of Riscani district, Chisinau decided to release Grigore Petrenko from house arrest, but he remained under judicial control, with a ban on leaving the place of his domicile.