The shooting inside a waiting room is the worst such incident to take place in the Czech Republic since 2015 when a man shot eight people dead at a restaurant in Uhersky Brod.
Six people were killed in a shooting in a hospital in the eastern Czech Republic on Tuesday, the prime minister said.
Police initially said the shooter was at large.
Video footage and pictures published by public radio later showed police arresting a person at the site but police didn't immediately confirm that it was the suspect.
Prime Minister Andrej Babis told Czech public television the shooting took place in a waiting room. The attacker opened fire at people's heads from close range, Babis said.
He said he is heading for the site.
Officials initially also published a photo of a man in a red jacket they said was the suspect, but later said that person was an important witness.
The shooting happened around 0600 GMT (7 am local time) on Tuesday morning in the University hospital in the city of Ostrava, located 350 kilometres (220 miles) east of Prague.
Officials say people have been evacuated from the clinic.
Police are boosting security across the country.