Jean-Pierre Bemba, the former vice-president of the Democratic Republic of the Congo, has been sentenced to 18 years in prison for rape and pillage committed by his troops, becoming the highest-level official to be sentenced at the international criminal court.

Bemba, 53, wearing a blue suit and tie, watched impassively from the dock during the hearing at The Hague on Tuesday.

The former militia commander is the third person convicted by the controversial “court of last resort” set up to try the world’s worst crimes in 2002.

Campaigners welcomed the lengthy prison term.

“Today’s sentencing marks a critical turning point for the thousands of women, children and men who were victims of Bemba’s orchestrated campaign of rape and murder,” said Karen Naimer, the director of the sexual violence in conflict zones programme at Physicians for Human Rights.

“The punishment meted out today can’t turn back the clock, but it can bring a measure of closure to those victims who’ve waited patiently more than a dozen years for this day to come,” she said.

The conviction was the ICC’s first verdict to recognise rape as a weapon of war and to employ the doctrine of command responsibility: that leaders are accountable for the crimes of their subordinates, the group said.

The court told Bemba that the years he had spent behind bars since his arrest in Belgium in 2008 and subsequent detention would be deducted from his sentence.

Earlier in the day Bemba’s lawyers said they would appeal against his war crimes conviction and press for a mistrial.

Bemba was found guilty in March of five charges of war crimes and crimes against humanity committed by his private army – the Congolese Liberation Movement (MLC) – after he sent them into the neighbouring Central African Republic (CAR) from October 2002 to March 2003 to put down a coup.

“I believe this is a very important day for international criminal justice, especially when it comes to sexual and gender-based crimes,” the chief prosecutor, Fatou Bensouda, told Agence France-Presse news agency at the time.

The prosecution had called for a minimum 25-year jail term.

In their ruling, three trial judges found that Bemba was responsible as the military commander of the MLC for a reign of terror by about 1,500 of his troops, including wide-scale rapes and murders, as they sought to quash a coup against the then CAR president Ange-Félix Patassé.

Bemba, who in 2002 became one of four vice-presidents of Congo under a peace agreement brokered by South Africa which ended the bloody civil war, was sentenced to two 18-year terms and two 16-year terms, to run concurrently.

The judges said Bemba could at any point have ended the MLC’s five-month rampage, but chose not to.

The ICC was set up in 2002 to be an independent international “court of last resort” for grave crimes that could not be dealt with locally.

The conviction and sentencing of Bemba will boost the court, which has a budget of more than $150m (£102m) annually.

Campaigners said the case was also historic because a record number of civilian victims – more than 5,200 – participated in the proceedings and may now be eligible for reparations.

However, analysts say the judgment will also highlight the failure of the court to punish anyone for the widespread and systematic human rights abuses committed by militia in Congo itself, particularly during the years of the civil war from 1998 to 2002.

“For the Congolese, it would feel like a missed opportunity for what happened there. One shouldn’t minimise what happened in the CAR but there is a sense of a missed opportunity,” said Hans Hoebeke, a senior analyst with the International Crisis Group in Nairobi, Kenya.

The institution has repeatedly been criticised for unfairly targeting Africa and African leaders. Critics point out that nine of the 10 “situations” currently being examined by the court relate to Africa.

Current high-profile ICC trials include that of Laurent Gbagbo, the former president of Ivory Coast, who denies charges that he orchestrated “unspeakable violence” in an attempt to hold on to power after losing an election in 2010, and that of Ahmad al-Faqi al-Mahdi, who is accused of razing medieval shrines, tombs and a 15th-century mosque that formed part of the Unesco world heritage site in Timbuktu, Mali, when the city was seized by Islamic militants in 2012.

In 2015, the ICC was forced to drop charges against Kenya’s president Uhuru Kenyatta, who had been accused of stoking ethnic violence after Kenya’s 2007 presidential election.

Prosecutors blamed their failure to put Kenyatta on trial on political interference and massive interference with witnesses, especially after Kenyatta was elected president in 2013. In April, charges against Kenyatta’s deputy, William Ruto, were also dropped for similar reasons.

In an interview with the Financial Times last week, Kofi Annan, the former United Nations secretary general, rejected accusations that the ICC was institutionally biased against Africans.

“I remind the Africans that it’s wrong for them to say that only African leaders are put into the dock,” Annan, who is from Ghana, said.

Several African governments have threatened to quit the ICC.

In February, the African Union backed a proposal by Kenyatta “to develop a road map for the withdrawal of African nations” from the court.

Last month, Yoweri Museveni, the president of Uganda, called the ICC useless during a swearing-in ceremony for his fifth term in power, prompting a walk-out by western diplomats.

Museveni has been named as a supporter of Bemba during Congo’s most recent civil war

Bensouda has said charges of bias are misplaced. “If certain people are looking to shield the alleged perpetrators of those crimes, of course they will say we are targeting [African states]. But … the victims deserve justice, the victims are Africans, and in the absence of the ICC nobody else is giving them justice,” Bensouda told the Guardian recently.

The ICC is also conducting “preliminary examinations” into eight further conflicts, including in Afghanistan, Colombia and Burundi, a spokesperson said.

The court’s decision has significant implications for politics in Bemba’s homeland.

Bemba finished second to the current president Joseph Kabila in the second round of the 2006 presidential elections with 42% of the vote. In 2007, hundreds of people died in the streets of Kinshasa when forces loyal to the two men clashed in the capital.

Bemba fled first to the South African embassy, then to Portugal and finally to Belgium, where he was arrested.

However, the wealthy businessman remains president of the opposition MLC and has retained a significant power base in the north and east of Congo, along with a presence in Kinshasa.

“His party still exists but is seriously diminished … The opposition at this point does not have a strong figurehead and there is no evidence of any significant mobilisation,” said Hoebeke.

Kabila’s second term expires in December, but polls are unlikely to be held as scheduled. Supporters of the president say more time is needed to organise logistics and prepare electoral rolls. Kabila’s critics accuse him of mounting a bid to remain in power indefinitely.

One leading opposition figure – Moise Katumbi – left Congo last month for medical treatment after being charged with mounting a coup attempt. Katumbi remains overseas, rallying opposition figures.