A spokesman for the ministry of health of the rival government based in the capital, Tripoli, told the BBC that 47 people were killed and more than a hundred people were injured in the blast, which was reportedly heard 60km (40 miles) away in Misrata.
Other news agencies have put the death toll at 50.
It is being reported that it was a water truck rigged with explosives which caused the bombing
State of emergency
The ministry of health has declared a state of emergency and called on all hospitals in Tripoli, 160km (100 miles) to the west of Zliten, and Misrata 60km (40 miles) to the east, to prepare to take in casualties.
Residents in Zliten have told the BBC that dozens of people are being transferred to Tripoli, as the hospital in Zliten is struggling to cope with the number of people injured.
Urgent calls for blood donations are being made to Zliten residents, the Lana news agency reports.
The BBC’s North Africa correspondent Rana Jawad says no-one has claimed responsibility for the latest attack, but there have been reports in recent months that IS affiliates are operating in that area.
The UN Special Representative to Libya, Martin Kobler, said that the blast was a suicide attack.
Libyan media said hundreds of recruits were outside performing morning exercises when the centre was targeted.
Libya’s al-Naba television network quoted the ministry of health of the rival rebel-led government as saying that a state of emergency had been declared, allowing hospitals in Tripoli and Misrata to receive the wounded.
In December, the country’s rival politicians signed a UN-brokered deal to form a unity government, but that has not yet been implemented.