Huge crowds have lined the streets of the capital of the Democratic Republic of the Congo to welcome home Jean-Pierre Bemba, the opposition leader whose war crimes convictions at the international criminal court (ICC) were quashed in June.

Bemba’s arrival is likely to trigger an intense new phase of political manoeuvring and instability in the country, which is due to hold presidential elections in December.

Candidates must be in the country to lodge their applications before a deadline in a week. Bemba is due to file his candidacy with the electoral commission on Thursday.

The 55-year-old left DRC in 2007 and spent 10 years in prison in The Hague before his surprise acquittal on appeal in June. The former warlord’s return will mobilise opposition to President Joseph Kabila, who has been in power since the assassination of his father in 2001.

“En route to the land of my ancestors, my homeland,” Bemba said on Twitter during the night.

En route vers la terre de mes ancêtres, ma patrie la République Démocratique du Congo #RDC #Bemba pic.twitter.com/jVKfaG8SCg — Jean Pierre Bemba (@bembajp) August 1, 2018

Thousands of supporters of the former rebel leader and vice-president thronged the streets of the capital, Kinshasa, to greet him. Police fired teargas at opposition supporters in the Limete neighbourhood as Bemba’s convoy approached, and witnesses said officers also fired teargas to clear the road near N’djili airport.

“In Bemba, we have a new hope and a new beginning,” said Eva Baizaba, the secretary general of Bemba’s MLC party, who met him off the plane along with about 300 other party members.

“The Congolese people have waited for this moment for a long time,” said Toussaint Bodongo, another party member. “Bemba will maybe bring the solution that we need to Congo.”

Kabila, whose second term expired in 2016, has repeatedly postponed elections. He is barred from a third term by the constitution and his close associates have denied that he hopes to find a way to stand again.

But the president was recently nominated as a candidate by his party’s youth league, and no one else appears to have been promoted as a possible replacement within the ruling coalition.

Bemba has a powerful support base in DRC and experts described his return as “a huge wildcard”.

“On his own, as an opposition leader, he could do extremely well,” Stephanie Wolters, an analyst based in South Africa told the Guardian when news of Bemba’s acquittal at the ICC was announced. “He is seen as a victim of Joseph Kabila’s politics and of international politics. If the opposition unite it would be very hard to see a Kabila victory that was anything but stolen.”

Supporters of former warlord Jean-Pierre Bemba gather in front of the party’s office in Gemena. Photograph: Junior D. Kannah/AFP/Getty Images

The opposition in DRC is fragmented, with neither of the two other main leaders – Félix Tshisekedi and Moïse Katumbi – committing to a formal coalition backing a single candidate.

Katumbi has applied for permission to return to DRC, possibly flying into Lubumbashi, in his stronghold in Katanga province, south-east DRC, in coming days.

The multimillionaire businessman faces court cases in DRC that he says are trumped up and has been in self-imposed exile in Europe.

A poll by the Congo Research Group at New York University shows roughly equal support – between 17% and 19% – for Bemba, Katumbi and Tshisekedi.

It put support for Kabila in an election at around 9%, though support from coalition allies could boost this considerably.

Bemba unsuccessfully opposed Kabila in elections in 2006. After his militia clashed violently with government forces in 2007, he was forced out of DRC and arrested in Belgium.

Judges at the ICC initially found Bemba guilty on five counts of war crimes and crimes against humanity committed by his private army during a five-month rampage in neighbouring Central African Republic in 2002.

Bemba had sent his militia, the MLC – a rebel force that he later transformed into a political organisation – into DRC’s northern neighbour to quash a coup against the then president Ange-Félix Patassé.

The 18-year sentence was the longest to be handed down by the court.

“Many people in the DRC have always seen his indictment for crimes in the CAR as politicised. He is the only politician of national stature to be sent to The Hague and is a lot more credible as an opposition leader than Katumbi or Tshisekedi,” said Wolters.

The attitude of regional powers will be key in coming months, with western powers seemingly without significant influence on Kabila.

Kabila has remained in power under a constitutional clause that enables a president to stay in office until his or her successor is elected.

DRC, which is rich in minerals but one of Africa’s most volatile countries, has never known a peaceful transition of power since it gained independence from Belgium in 1960 – and some experts fear the December elections will trigger a bloody conflict.

Additional reporting by Ignatius Ssuuna in Goma