( 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 . Bomb blast kills 8 at Nigerian medical school . Blast at soccer viewing center kills men , young boys . CNN 's Greg Botelho contributed to this report .
Police : 52 others injured , witnesses say . Security : Authorities stepped up security , surveillance around Abuja after blast . The explosion happened around 4 pm at International Plaza in Abuja 's Wuse II business district .