Police have discovered an abandoned lorry containing the bodies of 18 migrants who appeared to have suffocated.
Bulgarian authorities did not immediately give the nationalities of the migrants, but local media reported they were all from Afghanistan.
The truck was carrying about 40 migrants and the survivors were taken to nearby hospitals for emergency treatment, the Interior Ministry has said.
Bulgarian Health Minister Assen Medzhidiev said most of the survivors were in very bad condition, adding: "They have suffered from lack of oxygen, their clothes are wet, they are freezing, and obviously haven't eaten for days."
The truck was found abandoned along a highway near the capital, Sofia, and police were seeking to identify the lorry's drivers, who had fled.
Authorities discovered the passengers in a secret compartment below a hoard of timber.
Human Rights Watch previously said Bulgarian authorities are beating, robbing, stripping, and using police dogs to attack Afghan and other asylum seekers and migrants, then pushing them back to Turkey without any formal interview or asylum procedure.
“The European Union should ensure that Bulgaria immediately stops the illegal and dehumanising pushbacks at its borders and allows asylum seekers access to fair asylum procedures," said Michelle Randhawa, refugee and migrant rights officer at Human Rights Watch.
EU-member Bulgaria is a gateway to the bloc but is reticent to letting migrants in. The country has been trying to tighten security to stop a rising number of people seeking to cross the border.
The border is often the site of brutal attacks on refugees and last year the country was accused of shooting a refugee trying to reach the EU, after video emerged of a young Syrian man being fired at on the border with Turkey.
In a bid to stem the flow, Bulgaria has stepped up controls along the 145-mile barbed wire fence covering almost the entire border with Turkey.
Austria and the Netherlands have blocked Sofia's bid to join the Schengen border free zone.
In October 2019, British police found the bodies of 39 people inside a refrigerated container that had been hauled to England.
Police said all the victims, who ranged in age from 15 to 44, came from impoverished villages in Vietnam and were believed to have paid smugglers to take them on a risky journey to better lives abroad.
