(bbc) -- According to the government, an army officer took a police helicopter and flew it over the capital, Caracas.
Footage on social media shows a helicopter swooping over the city followed by a loud bang.
The officer believed to have seized the aircraft has issued a video statement condemning the government.
There are no reports of anyone being killed or injured.
There have been almost daily protests against Mr Maduro's leftist government for more than two months, as the country's economic and political crisis has worsened.
More than 70 people have been killed in protest-related violence since 1 April, according to figures released by the chief prosecutor's office.
President Maduro said grenades had been dropped in the attack and he promised that security forces would capture those behind it.
"I have activated the entire armed forces to defend the peace," he said in an address from the Miraflores presidential palace.
"Sooner or later, we are going to capture that helicopter and those who carried out this terror attack."
The officer, who called himself Oscar Perez, appeared on Instagram, flanked by armed and masked men in uniform.
He appealed to Venezuelans to oppose "tyranny".
"We are a coalition of military employees, policemen and civilians who are looking for balance and are against this criminal government," he said.
"We don't belong to any political tendency or party. We are nationalists, patriots and institutionalists."
He said the "fight" was not against the security forces but "against the impunity of this government. It is against tyranny".