Tbilisi (AFP) - Georgia was investigating Wednesday the attempted murder of an opposition lawmaker whose car exploded in central Tbilisi just days before a key parliamentary poll.

Givi Targamadze, who is running for office for the ex-Soviet republic's main opposition United National Movement (UNM) party, told journalists that an explosive device planted in the back of his car detonated on Tuesday night.

Targamadze, who was sitting in the front of the vehicle, was unhurt.

Four passers-by were rushed to hospital, one with serious but not life-threatening injuries, Health Minister David Sergeyenko said in televised comments.

Prime Minister Giorgi Kvirikashvili on Wednesday condemned the blast as "sabotage against the state" committed by the country's "enemies" who "will be held accountable to the fullest extent of the law".

Police launched an investigation into what deputy interior minister Besik Amiranashvili called "attempted murder in aggravating circumstances".

UNM leader David Bakradze said that "no matter who was behind the car attack, the authorities are responsible for creating a climate of hatred in which opposition politicians are being attacked".

The bombing was not the first violent incident to occur in the lead-up to the vote.

On Sunday, unknown assailants fired shots during a campaign rally held by MP candidate Irakli Okruashvili in the central city of Gori, injuring two men.

- 'Moment of truth' -

Saturday's knife-edge parliamentary election will see the UNM and the ruling Georgian Dream party clash in a bitter power struggle.

The two pro-Western parties are neck and neck in opinion polls and have equal chances of forming Georgia's next government.

Georgia's ex-president and UNM's founder Mikheil Saakashvili -- who lives in exile in Ukraine -- addressed via videolink tens of thousands of supporters who gathered in downtown Tbilisi for a campaign rally.

"Just a few days separate us from our (electoral) victory," he told crowds of people chanting his name.

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"Georgia is approaching a historic moment, a moment of truth."

Saakashvili, who serves as a regional governor in Ukraine, is wanted by Georgian prosecutors for alleged abuse of office during his nine-year rule -- charges he has denounced as trumped-up and politically motivated.

The UNM has accused the Georgian Dream government of orchestrating a political witch-hunt after members of Saakashvili's team were probed and some jailed since the Georgian Dream party defeated UNM in parliamentary and presidential elections in 2012 and 2013.

Western countries voiced concerns over what they perceive as a pattern of persecution of political opponents.

Targamadze was featured in a 2012 Russian television film which alleged he and a group of Kremlin critics plotted attacks in Russia.

The film led to a court case that saw several Russian opposition members jailed. Targamadze himself is wanted in Russia.