Police arrested four people during checks at the Summer Carnival in central Rotterdam after finding several real and fake firearms, Rijnmond reports. In the meantime, nine people were taken to the hospital, and 19 were treated on-site after becoming unwell amid heat and crowds, a Rotterdam Safety Region spokesperson said. Some of the people fainted.

The Rotterdam Summer Carnival street parade started at noon Saturday on the Blaak. Drawing on carnival traditions from the Antilles, Cape Verde, Suriname, and Spain, the event attracts tens of thousands of people each year.

The nine sick people were transported to the hospital at around 7:30 p.m. The spokesperson described “quite a few cases of people becoming unwell” at this year’s event—more than last year.

“It is warm and crowded in the city,” the Safety Region spokesperson said on Saturday. “Take good care of yourself and drink enough water.”

On the Schiedamsedijk, officers checked a group of young people. Two boys left the group and walked away, drawing officers’ attention. After a short chase, police stopped them. They had one real firearm and one fake firearm. The two were arrested and taken to the police station.

Checks were also carried out on the Lijnbaan, where one real firearm and one fake firearm were found. Two people were arrested there, bringing the total to four.