In an emotional speech, Kyaw Moe Tun said no-one should co-operate with the military until it handed back power to the democratically elected government.
Security forces intensified a crackdown on anti-coup protesters on Saturday.
Local media say dozens were arrested, and that a woman was shot in the city of Monwya. Her condition is not clear.
The country has been rocked by protests since top government leaders, including Aung San Suu Kyi, were overthrown and detained after the army took power on 1 February.
What did the UN ambassador say?