Win Htein, 79, is a senior member of Ms Suu Kyi’s ruling National League for Democracy (NLD) party, and was arrested from his daughter’s house during a midnight raid, the party’s office confirmed on Friday, adding that he has been held in Naypyidaw police station.
Mr Htein, a long-time confidante of ousted Ms Suu Kyi, had publicly called for civil disobedience to protest against Monday’s coup.
The politician had already spent around 20 years in prison when he was first arrested in 1989 for campaigning against military rule.
The military-run government on Thursday ordered internet providers to temporarily block Facebook, Instagram and Whatsapp for millions of people in the country.
Meanwhile, on Friday, hundreds of students and teachers staged protests against the coup and raised slogans, chanting: “Our arrested leaders: release now, release now”.
During this week, doctors and other protesters have adopted the three-finger salute, seen previously in protests in Hong Kong and Thailand, as a gesture of resistance against power inspired by The Hunger Games movies.
About 300 students took part in demonstrations at the Dagon University on the outskirts of Yangon and chanted “Long live Mother Suu” and “we want democracy”.
“We will not let our generation suffer under this kind of military dictatorship,” a student in the demonstrations, Min Sithu, told the news agency AFP.
At least 133 civilian government politicians or officials and 14 activists have been detained by the military in its crackdown since the small hours of Monday morning, according to the Assistance Association for Political Prisoners.
International leaders have condemned the coup, with UN secretary-general Antonio Guterres appealing to other countries not to allow it to succeed.
In the latest statement, newly elected US president Joe Biden called on the Burmese military to “relinquish power” and release all the leaders detained.
“There can be no doubt, in a democracy, force should never seek to overrule the will of the people or attempt to erase the outcome of a credible election,” Mr Biden said on Thursday in what was his first major foreign policy intervention.
The British government also warned Myanmar it is considering “next steps” as foreign secretary Dominic Raab condemned the treatment of Nobel laureate Ms Suu Kyi.
The 1 February coup unfolded a day before Myanmar’s parliament was scheduled to host the swearing-in ceremony of members elected during the November general election, in which the NLD won by a landslide.
The party backed by the military performed badly, after which the armed forces claimed without evidence that there was widespread rigging. Military leaders later said they could not rule out seizing power if the election commission failed to investigate their complaints.
