The arrest follows that of Suu Kyi and Myanmar president Win Myint who were detained on Monday as the military seized the levers of government, granting army chief Min Aung Hlaing control of the country.

The move ended Myanmar's 10-year dalliance with democracy after decades of junta rule.

Suu Kyi's National League for Democracy (NLD) said through a verified Facebook page that party stalwart Win Htein had left Naypyidaw on Thursday afternoon, and gone to Yangon.

"He was arrested from his daughter's house where he was staying at midnight (in Yangon)," party press officer Kyi Toe said, adding he was being held in a Naypyidaw police station.

The 79-year-old is a longtime political prisoner, who has spent long stretches of time in and out for detention for campaigning against military rule.

Considered Suu Kyi's right-hand man, he has long been sought out by international and domestic media for insights into what Myanmar's de facto leader is thinking.

Ahead of his arrest, he had told local English-language media that the military putsch was "not wise", and that its leaders "have taken (the country) in the wrong direction".

"Everyone in the country should oppose as much as they can the actions they are seeking to take us back to zero by destroying our government," he told Frontier Myanmar in the coup's aftermath.

Suu Kyi has not been seen in public since Monday.

According to the Assistance Association for Political Prisoners, a Yangon-based group that monitors political arrests in Myanmar, more than 130 officials and lawmakers have been detained in relation to the coup.

Telecoms providers in the country have also been ordered to throttle Facebook, the main means of accessing the internet and communicating for millions of people in Myanmar.

- 'Relinquish power' -

The putsch has drawn condemnation globally and on Thursday, US President Joe Biden reiterated his call for the generals to reverse course.

"The Burmese military should relinquish power they have seized, release the advocates and activists and officials they have detained, lift the restrictions in telecommunications, and refrain from violence," Biden said.

He spoke hours after his national security adviser Jake Sullivan said the White House was "looking at specific targeted sanctions both on individuals and on entities controlled by the military that enrich the military." He did not give further details.

So far, no large-scale protests have emerged on the streets of Myanmar, though small pockets of dissent have popped up, with medical doctors choosing to wear reb ribbons -- NLD's colours.