Khin Maung Latt, 58, was arrested late Saturday night and died, according to a military doctor, of heart problems.
But pictures of the body show injuries to the head and back.
According to former MP Ba Myo Thein, the party chairman was likely subjected to severe torture in captivity.
-It is completely unacceptable, says Ba Myo Thein.
"Hanged upside down"
According to witnesses, several other party representatives were arrested on Sunday night.
Police and the military also tried to arrest the NLD's intelligence chief Maung Maung.
- Maung Maung's brother was beaten by police and military, he was hung upside down and tortured because they could not find anyone to arrest, says a former member of parliament.
Nine unions went on strike on Sunday, starting on Monday, in protest against the military junta.
Around the country, tens of thousands of protesters continued to protest against the military regime and to demonstrate in support of the elected leader, imprisoned leader Aung San Suu Kyi.
Police fled
At the same time, there are reports of defections within the power apparatus.
The authorities in Myanmar have over the weekend asked India to hand over eight police officers who fled to the neighboring country after the coup.
According to the UN, at least 54 people have been killed since the military coup on February 1, when Aung San Suu Kyi and a large number of other civilian politicians were placed under house arrest.
The takeover and the brutal violence against civilians have been condemned by the UN and a large number of countries.
According to UN Human Rights Director Michelle Bachelet, more than 1,700 people, including 29 journalists, have been arrested since the coup.