The gunman is believed to be a veteran of Croatia's war of independence in the 1990s. Police officials said that he used an unregistered gun.
An assailant entered an elderly care home in the central Croatian town of Daruvar, killing at least six people and injuring another six, authorities said.
Croatia's police chief, Nikola Milina, said five people died immediately, while one more person died in a hospital.
The victims were five residents of the care home and one employee, Milina said.
Police said they were informed after 10 am Monday that a man using firearms killed and wounded several people at the nursing home.
The suspect fled the scene, but the police soon caught him in a cafe in downtown Daruvar.
"The person suspected of the crime has been apprehended," the Bjelovar-Bilogora County Police said in a statement.
Most of the wounded were brought in with severe injuries, according to the head of the hospital in nearby Pakrac. Four of the injured had head trauma and gunshot wounds to the chest, the hospital's surgeon, Branko Sudar, said.
The private home housed around 20 people, according to the Daruvar Mayor Damir Lneniček.
Government reels in shock
The mass shooting in Daruvar has shocked the Adriatic nation of some 3.8 million and its leaders.
President of Croatia Zoran Milanović condemned what he said was a "savage, unprecedented crime".
"It is a terrifying warning and a last call to all competent institutions to do more to prevent violence in society, including even more rigorous control of gun ownership," Milanović said in a post on Facebook.
"We are shocked. It's a truly monstrous act, the murder of multiple people, of (the assailant's) mother and older people mostly born in the 1930s," Prime Minister Andrej Plenković said at a press conference in the seaside city of Split.
“We had some difficult situations, but I don't remember this many people being killed (in one attack),” he added.
State prosecutor general Ivan Turudić, who came to Daruvar after the attack, did not comment on whether Croatia needs more stringent gun laws, but he did state that "it has to become even more clear that perpetrators will be found, arrested and prosecuted."
Authorities are investigating the motive behind the attack. The gunman is believed to be a veteran of Croatia's war of independence in the 1990s, however. Police officials said that the assailant used an unregistered gun.
Daruvar is a spa town and municipality in the region of Slavonia, with a population of about 8,500 people.