Hungarian police have taken into custody the captain of a cruise ship involved in a crash on the Danube River in the capital, Budapest, which killed at least seven South Korean tourists and left more than 20 other people missing.

In a statement issued on Thursday, authorities said the man – a 64-year-old Ukrainian citizen – was detained after questioning for alleged reckless misconduct in waterborne traffic leading to mass casualties.

In line with Hungarian laws, the suspect was identified only as Yuriy C., referencing his first name and the initial letter of his last name.

The suspect was in charge of a tourist boat which collided with another sightseeing vessel, the “Mermaid”, on Wednesday near the Hungarian parliament building. The “Mermaid” then turned over on the river, which has been flooding with very strong currents while a rainstorm enveloped Budapest.

Police showed security camera footage from a bridge that showed the two boats colliding. “It sank within seven seconds,” police colonel Adrian Pal told a news conference.

So far, only seven of the 35 people onboard, that included 33 South Koreans, are known to have survived the incident, with 21 others still missing.

A massive rescue effort was under way with boats, divers, spotlights, and radar scanning along the river several kilometres downstream.

Officials said 14 people had been pulled out of the water so far. Seven of them had died, with the other seven suffering from hypothermia but stable.

Staff from the South Korean embassy in Budapest were assisting Hungarian officials in identifying those rescued and the deceased.

Dozens of emergency vehicles were visible on a stretch several kilometres downstream from the site of the accident, which happened shortly after 9pm (19:00 GMT).

The hours that have passed since the accident make it less likely that new survivors will be found in the central Budapest area, as the strong currents have carried people far downstream, emergency rescue chiefs told state media.

The National Ambulance Service was conducting searches along a stretch downstream from Budapest and on alert on the entire Danube stretch south of Budapest in Hungary, where all boat traffic has been halted.

Television footage showed the bank of the Danube closed off by police on the Pest side, across from the World Heritage site of Buda Castle.

The Danube’s flooding and currents made rescue efforts extremely difficult, a rescue diver told the state broadcaster, adding that the waters of the Danube were only about 10-12 degrees Celsius.

The boat was is described on the sightseeing company’s website as “one of the smallest members of the fleet”. It had two decks, a 150-horsepower engine and a capacity for 60 people, or 45 for sightseeing cruises.

A shipping expert told state television that it was likely that the capsized boat had collided with a very large vessel that had sunk it very quickly. The hull is yet to be found on the riverbed.