Media in Libya said the attack struck the al-Jahfal training camp.
The training centre had been a military base during the rule of ousted Libyan leader Muammar Gaddafi.
Libya has been hit by instability since his overthrow in 2011, and there is concern Islamic State (IS) militants are gaining a foothold there.
The Lana news agency, run by the internationally-recognised government, said at least 50 people were killed and 127 injured in the blast, which was reportedly heard 60km (40 miles) away in Misrata.
Urgent calls for blood donations are being made to Zliten residents, the agency said.
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.
Source:3news.com
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