Casualties have been taken to a hospital in Misrata 60km (40 miles) away. Reuters/BBC News
At least 47 people have been killed by a truck bomb targeting a police training centre in the western Libyan city of Zliten, reports say.
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 country has been run by two governments - only one of which is recognised by the international community.
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 100 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.
This is one of the deadliest bomb attacks in Libya and comes at a time when hospitals across the country are suffering from severe shortages in medical supplies.
The explosion is similar in size and impact to the multiple bombings in the eastern town of al-Qubbah that targeted a petrol station in February last year.
That incident was claimed by the so-called Islamic State group, just a few months after it established a foothold in the country.
The latest explosion has not been officially claimed by any group yet, but it comes against the backdrop of intensified attacks this week carried out by IS militants near Libya's eastern oil ports.
If nothing else, the latest bombing is an indication of how the political and military chaos is degrading the institutional capabilities in the country.
Devastating attacks of this kind are on the rise, as the ability to prevent them and deal with the aftermath is at an all-time low.
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 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 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.
In December, the country's rival politicians signed a UN-brokered deal to form a unity government, but that has not yet been implemented.