A legal adviser for Myanmar's ruling National League for Democracy (NLD) party has been shot dead outside Yangon's international airport, in what appeared to be a rare outbreak of political violence in the commercial capital.

Police arrested a lone gunman, but a motive was unknown in the killing of 65-year-old Ko Ni on Sunday, a prominent member of Myanmar's Muslim minority.

A taxi driver who tried to stop the gunman was also killed, according to Zaw Htay, spokesman for President Htin Kyaw.

"We have detained and are questioning the gunman to find out why he killed him, and who is behind it or paid him to do it," Zaw Htay told Reuters.

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A police official told Reuters the suspect was a 53-year-old Myanmar citizen from the central city of Mandalay.

Mourners gathered in front of the home of Ko Ni on Monday.

"This is a crime and I request you find justice," senior party member Tin Oo told media, saying Ko Ni's death was a loss for the whole country.

The apparent assassination comes amid heightened tensions in Buddhist-majority Myanmar, where leader Aung San Aung San Suu Kyi is under pressure over a heavy-handed security operation in an area of the country's northwest that is populated mostly by Muslims.

'Talking to his grandson'

Ko Ni had just embraced his young grandson as he stepped out of the airport terminal on his return from Jakarta, said the lawyer's daughter Yin Nwe Khine.

"My father was talking to his grandson. Then, I heard a gunshot. At first, I thought it was a car tyre blowing out, then I saw my father lying on the ground," she said.

Ko Ni, an expert in constitutional law, had spoken out about the powerful role the military retains in governing Myanmar, despite handing over power to Aung San Suu Kyi's civilian administration in April.

"My father was often threatened and we were warned to be careful, but my father didn't accept that easily. He always did what he thought was right," said Yin Nwe Khine.

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"A lot of people hate us because we have different religious beliefs, so I think that might be why it happened to him, but I don't know the reason."

Ko Ni had joined Minister for Information Pe Myint on the visit to Muslim-majority Indonesia, billed as an opportunity to share experiences of national reconciliation.

The delegation included several Myanmar Muslim leaders, some belonging to the mostly stateless Rohingya minority.

Yanghee Lee, the UN's special rapporteur for Myanmar, voiced her outrage over Ko Ni's killing, saying she had met him on her last trip to the country earlier this month, which included a visit to Rakhine.

"My deepest and most sincere condolences to the family of U Ko Ni the most prominent and respected Muslim lawyer of Myanmar," she tweeted, calling on Aung San Suu Kyi's government to "get to the bottom" of his death.

Source: News agencies