(CNN) -- An explosion Wednesday afternoon rocked a parking lot at a crowded plaza in Nigeria's capital, killing 21 people and injuring 52 others, authorities said.
The blast happened about 4 p.m. in Abuja's "ever-busy Emab Plaza ... at the peak of business," said Ezekiel Manzo, spokesman for Nigeria's National Emergency Management Agency.
About 40 vehicles were destroyed, in addition to the human toll, he said. In a news release posted online, the Nigerian national police force said the injured were being treated at five hospitals.
It was not immediately clear who was responsible for the attack.
Ibrahim Abdullahi told CNN he was in a nearby office when he heard a "loud and terrifying sound."
"When I looked from the window of my office, I saw thick, black smoke envelope the sky," Abdullahi said. "I could see the raging fire from the blast."
Olumide Sanya said the blast shook the nearby building that he was in. "Windows, doors were shattered."
The area was evacuated and security personnel managed to take control of the scene, according to Manzo.
This was the first part of a wider effort to beef up security around Abuja. The national police's inspector general ordered stepped-up security and surveillance around the capital as part of efforts "to improve overall public safety."
"(Authorities) call on the members of the public to remain vigilant and to stay away from scenes of explosion(s) so as not to fall victim in a likely case of (a) secondary blast," police said.
Nigeria has seen significant violence -- most of it centered in northern parts of that African nation and blamed on Boko Haram, an Islamist militant group with a stated aim of imposing strict Sharia law across the country.
This includes an attack earlier this month in the Borno state village of Kummabza in which militants killed 30 males and abducted 60 females, some of them girls between the ages of 3 and 12.
CNN's Greg Botelho contributed to this report.