Myanmar’s junta has executed four prisoners including a former politician and a veteran activist, drawing shock and revulsion at the country’s first use of capital punishment in decades.

Junta-controlled media reported on Monday that four men, including Phyo Zeya Thaw, a rapper and former lawmaker from Aung San Suu Kyi’s party, and the prominent democracy activist Kyaw Min Yu, known as Jimmy, had been executed. They were accused of conspiring to commit terror acts and were sentenced to death in January in closed trials.

The UN special rapporteur Thomas Andrews said he was “outraged and devastated” by the executions. “The widespread and systematic murders of protesters, indiscriminate attacks against entire villages, and now the execution of opposition leaders, demands an immediate and firm response by member states of the United Nations,” he said.

France and Japan also condemned the executions, while the US national security council described the killings as “heinous”.

Aung Myo Min, the human rights minister in Myanmar’s national unity government (NUG), which was formed in exile by elected politicians, ethnic minority representatives and activists, said he was extremely saddened. “What else do we need to prove how cruel the murderous Myanmar’s military is?” he said.

Following reports of the executions, demonstrators in Yangon held up a banner which read “we will never be frightened”.

Another banner was hung on a bridge in Yangon bearing a warning that the junta should “be ready to pay for the blood debt”. Text underneath read: “RIP Zeyar Thaw, Jimmy, Hla Myo Aung and Aung Thura.”

Local media reported that the families of the men had travelled to Insein prison in Yangon demanding to see their loved ones’ bodies.

A source close to the family of Kyaw Min Yu said they had received confirmation from the deputy prison chief of Insein prison that the death penalty had been carried out. No information was provided about when the execution took place. Prison officials refused to hand the body over to relatives, they said, despite prison regulations stating it must do so unless there is a special reason.

Myanmar’s military junta seized power in a coup in February 2021, ousting the elected government of Aung San Suu Kyi, and has since unleashed a campaign of brutal violence to suppress opposition.

A total of 14,847 people have been arrested since the coup, while 11,759 remain in detention, according to the advocacy group Assistance Association for Political Prisoners (AAPP) Burma, which monitors arrests and killings.

According to AAPP Burma, 76 prisoners have been sentenced to death since the coup, including two children. A further 41 people have been sentenced to death in absentia. Before the executions on Monday, Myanmar had not carried out capital punishment in more than 30 years, according to the UN.

Many in Myanmar turned their social media profile pictures black and red in a show of mourning. Others posted lines from the men’s lyrics and speeches including the line “nothing’s gonna happen if we are all united” from one of Phyo Zeya Thaw’s rap songs.

Phyo Zeya Thaw, 41, was arrested in November when about 100 police and soldiers raided a housing complex in Yangon. Before entering politics he was an activist and rapper. In 2000 he released the country’s first rap album, having founded the hip-hop band Acid. His lyrics, and their thinly veiled criticisms of the previous military regime, captured the anger and frustrations of a generation of young listeners.

Phyo Zeya Thaw was an activist with the movement Generation Wave, which used graffiti, pamphlets and coded protest material to campaign against the previous regime. Like many of the group’s members, he was arrested and imprisoned. He went on to become a lower house MP in April 2012, the same year Aung San Suu Kyi was elected to parliament.

Kyaw Min Yu, 53, a veteran activist, was arrested in an overnight raid in October. He was a prominent leader of the 88 Generation Students Group, which led pro-democracy uprisings against the military, and was imprisoned in 1988 for his role in the protests. He was released in 2005 but jailed again from 2007 until 2012.

Kyaw Min Yu was also a writer, and while in prison translated works including Dan Brown’s The Da Vinci Code and Angels and Demons, and wrote the novel, The Moon in Inle Lake. His 2005 self-help book Making Friends was a bestseller, according to PEN International.

The two other executed men – Hla Myo Aung and Aung Thura Zaw – were accused of killing a woman they suspected was a military informer in Yangon, according to Agence France-Presse.

The men had tried to appeal but their sentence was upheld in June. They were reportedly denied access to legal counsel during their appeal, in violation of international human rights law.

The junta-controlled newspaper Global New Light of Myanmar said on Monday the men had given “directives, made arrangements, and committed conspiracies for brutal and inhuman terror acts”.

The junta gave no details on how the men were executed.

Yee Mon, the NUG’s defence minister, wrote on Facebook: “This revolution isn’t over until we bring justice against [junta chief] Min Aung Hlaing. He won’t have an inch of earth to run for, [we] will chase him down until the end of the world.”