A sound bomb caused panic on the Cairo metro this morning, Al-Ahram Arabic news website reported.
Sixteen people were transferred to hospital with minor injuries, said an official at the ambulance service.
The number has risen to more than twenty injured, as announced later by the Health Ministry.
The injuries were caused when people tried to escape from the site of the blast, he added.
The incident occurred while the metro was departing from Helmeyet Al-Zaytoun station on the Marg to Helwan line.
Metro movement stopped completely and the train has been moved to Saray Al-Qobba station.
Authorities are investigating the incident.
Cairo has seen a wave of militant attacks over the past year, particularly on police and army targets.
Several explosive devices have previously been found or detonated in metro stations, but none were major or caused casualties.
Sound bombs were tossed by unidentified suspects on Tuesday in Sayeda Zeinab and Zahraa El-Maadi metro stations, causing no casualties, said an interior ministry official.
Metro stations have also been shut several times in the past year due to various anonymous threats or anticipated demonstrations by supporters of ousted president Mohamed Morsi.
On Saturday, no first-line trains stopped at Saray Al-Qobba station due to security concerns.