Colombia's government declared three days of mourning after at least 21 people died in a car bomb at a Bogota police cadet training academy and 68 were wounded on Thursday-- the worst such incident in the city in 16 years.

Authorities blamed leftist National Liberation Army (ELN) rebels for the attack.

The defense ministry said the "terrorist act" was carried out using a vehicle packed with 80 kilograms (around 175 pounds) of explosives.

"Unfortunately, the preliminary toll is 21 people dead, including the person responsible for the incident, and 68 wounded," Colombian police said in a statement, adding 58 of those injured had been discharged from hospital.

"All Colombians reject terrorism and we're united in fighting it," President Ivan Duque tweeted in the aftermath.

Later in a statement to the nation, he said he had ordered reinforcements to Colombia's borders and routes in and out of cities.

The bomber -- who authorities confirmed was killed in the attack -- struck at the General Francisco de Paula Santander Officer's School in the south of Bogota during a promotion ceremony for cadets.

No group has claimed responsibility, but public prosecutor Nestor Humberto Martinez named suspect Jose Aldemar Rojas Rodriguez as the "material author of this abominable crime."

Martinez said Rojas Rodriguez entered the school compound at 9:30 am (1430 GMT) driving a grey 1993 Nissan Patrol truck, but gave no details about the explosion.

Defense Minister Guillermo Botero said he had "full evidence" that Rojas Rodriguez, 56, has been a member of the ELN for more than 25 years.

'BRUTAL ACT OF TERRORISM'

Ecuador's President Lenin Moreno said one of the dead was an Ecuadoran cadet, while a second suffered light injuries.

"The brutal act of terrorism in Bogota took the life of a compatriot," Moreno said on Twitter.

"My sincerest thoughts go to the family, friends and companions of Erika Chico."

Meanwhile, Panama's President Juan Carlos Varela said that 45 Panamanian cadets were present during the attack, with two injured.

Fanny Contreras, the Colombian armed forces' health inspector, told local radio that the truck "entered (the school compound) suddenly, almost hitting the police, and then there was the explosion."

Carol Oviedo said her brother Jonathan, a cadet, told her on the phone he had been injured, before the connection was cut.

"In two years since he joined the police, he's never had to face a situation like this," she said.

Like other families, she was lingering in the vicinity of the academy hoping to hear some news.

United States assistant secretary of state in charge of Latin America, Kimberly Breier condemned the attack and said: "Our condolences and sympathies go to the victims and family members of those killed."

The US embassy in Bogota offered its "help in investigating this reprehensible attack."