Myanmar’s major trade unions called on members to shut down the economy from Monday to back a campaign against last month’s coup, raising pressure on the junta as its forces fired weapons and occupied hospitals in the main city Yangon after a day of massive protests.

Witnesses reported sounds of gunfire or stun grenades in many districts of the commercial capital after nightfall, as soldiers set up camp in hospitals and university compounds, local media reported. It was not clear whether anyone was hurt...