The Interfax news agency cited Armenia’s emergency service as saying the explosion occurred in a building at the Surmalu retail market, about two kilometers (1.2 miles) south of the centre of Yerevan, where fireworks were sold.
The cause of the blast was not immediately known.
The emergency situations ministry said there were 10 firefighting trucks on the spot and another 10 were on their way to the market.
Photos and videos posted on social media showed a thick column of black smoke over the market and successive detonations could be heard.
Russia’s state news agency Tass cited the city’s mayor as saying an unspecified number of people were trapped in rubble.