Explosions echoed across central Harare and soldiers took over the headquarters of state broadcaster ZBC in a dramatic escalation of a dispute with President Robert Mugabe over political succession.
Some ZBC members of staff were manhandled when soldiers occupied the premises in the early hours of Wednesday, sources told Reuters.
Staff were told they "should not worry" as the soldiers were merely there to protect the site, one source said.
The reason for the blasts heard in the capital was not immediately apparent.
Earlier this week, Defence Force commander General Constantino Chiwenga warned Mr Mugabe the military would "step in" if the president continued to expel supporters of the sacked vice president from the ruling party.
The former vice president, Emmerson Mnangagwa, had long been tipped as most likely successor to the 93-year-old president, but Mr Mugabe's 52-year-old wife Grace is now vying for the job.
A Reuters reporter witnessed six armoured personnel carriers on thoroughfares on the outskirts of the capital, Harare, and soldiers told passing cars to keep moving.
The presence of troops, including the movement of armoured vehicles from a barracks northwest of Harare earlier in the day, sparked rumours of a coup against Mr Mugabe, although there was no evidence to suggest Zimbabwe's leader of the last 37 years had been toppled.
Isaac Moyo, Zimbabwe's ambassador to neighbouring South Africa, dismissed the talk, saying the government in Harare was "intact" and blaming social media for spreading false information.
"There's nothing really happening. They are just social media claims," Mr Moyo said.