The blast took place at al-Jahfal police training centre in the western Libyan city of Zliten, reported BBC.
The training centre served as military base during the leader Muammar Gaddafi regime.
Instability continues to hit Libya since 2011 when Gaddafi was overthrown and ISIS has also gained foothold in the country.
"The blast was a suicide attack," said Martin Kobler, the UN Special Representative to Libya.
Hundreds of recruits were inside the centre when blast took place.
State of emergency has been declared after the blast. In December, Libya's rival politicians signed a UN-brokered deal to form a unity government, but that has not yet been implemented.