The crash happened about 6 a.m. near the town of Slavonski Brod on the highway between the Croatian capital of Zagreb and the Serbian border, a key artery over the summer due to tourists and workers coming home from Western Europe.
Police said the bus had Kosovo license plates and was traveling from Frankfurt, Germany, to Kosovo''s capital of Pristina, which is south of Serbia.
Officials said the bus was carrying 67 passengers, including children, and two drivers, one of whom died in the crash. The 44 injured were transferred to local hospitals. Slavonski Brod hospital chief Josip Samardzic said eight people had serious injuries.
Authorities said the bus driver was detained after he apparently lost control of the vehicle after briefly falling asleep.
“He said he fell asleep for a moment,” local deputy prosecutor Slavko Pranjic said, according to Index news site in Croatia.
Police said the bus slid off the road into the grass before flipping on its side.
One passenger, Ramo Gashi, told state HRT television that “something burst.”
“I saw, in a split second, all these people, the entire meadow, the channel below, behind the motorway,” he said. “I saw the wounded, the dead, I saw everything."
Croatian Prime Minister Andrej Plenkovic expressed “sadness and grief” and extended his condolences to the victims'' relatives and the people of Kosovo.
Croatian President Zoran Milanovic also expressed condolences and wished a speedy recovery to the injured.
Kosovo President Vjosa Osmani, who cut short her stay in Tokyo at the Olympic Games because of the crash, extended her sorrow in a message on Facebook and declared Monday a national day of mourning in Kosovo.
“With our heart and in spirit, we are close to the families who lost their loved ones in this tragedy," Osmani said. “It is an indescribable pain and a great loss."
Kosovo Prime Minister Albin Kurti said “this is a tragic day for our country and for our people.”
He arrived in Croatia later Sunday and visited the injured in the hospital with Croatia''s prime minister.
“We are here to be with our citizens,” Kurti said, according to Index. “Hundreds of citizens who live outside Kosovo, they come to Kosovo and our economy depends on this.”
Traffic on the highway was halted for hours before the bus was removed and one lane was reopened.