SEOUL, South Korea — Survivors of a tour boat which sank in the Danube River in Budapest say they barely managed to grab onto a drifting lifeboat while watching in horror as others around them struggled and shouted for help in the dark, rainy waters.

Only seven of the 35 people on the boat, including 33 South Koreans, were rescued. Seven others have been confirmed dead and 21 are missing.

Survivors said the small sightseeing boat had almost finished an hourlong night tour of the Hungarian capital on Wednesday and was nearly at its stop when a larger cruise ship hit it under a bridge. They said about 20 people were on the deck taking photographs or preparing to disembark. The others were in the cabin.

"I saw that big cruise ship coming closer to us but I never imagined it would ram our boat," a South Korean surnamed Jeong was quoted as saying by the Yonhap news agency.

Jeong, 31, said she and others on the deck were thrown into the cold Danube waters by the impact of the collision. Police said it took only seven seconds for the boat to overturn and sink.

She said she saw a lifeboat drifting near her and managed to get hold of it. She threw a rope to another South Korean tourist surnamed Yoon, who was near her.

"Our boat was turned over in an instant and began sinking," Yoon, 32, told Yonhap. "All those on the deck fell into water and I think those staying in the cabin on the first floor probably couldn't get out of the ship swiftly."

While holding onto the lifeboat together, Jeong and Yoon said they shed tears when they saw the heads of other people coming up and down in the fast-moving river.

"The people had been plunged into the river in the darkness and shouted, 'Help me!' while floundering in the waters. But I couldn't do anything for them," Jeong said, crying.

Candles and flowers are left under the Margaret Bridge in Budapest, Hungary, near where a tour boat sank. Zoltan Balogh / AP

Another survivor surnamed Ahn, 60, said a crew member of another sightseeing boat sailing nearby extended a hand to him after he was tossed into the river. But he lost his grip and was carried away by waters before he got hold of a drifting plastic object.

Yoon said she saw the cruise ship that rammed her boat keep sailing without stopping to help with the rescue after the collision.

Police launched a criminal investigation and detained and questioned the Ukrainian captain of the cruise ship late Thursday. The 64-year-old man is suspected of endangering water transport leading to a deadly mass accident.

The Viking Sigyn collided with the tour boat on the Danube River. Bernadett Szabo / Reuters

Hungary's state TV reported that all of the seven rescued people have been released from the hospital except one who is being treated for broken ribs.

Thirty South Korean tourists were aboard the sightseeing boat along with two South Korean guides and a photographer. Two Hungarian crew members are among the 21 missing.

It was still unclear what exactly caused the collision. A preliminary investigation showed that none of the South Koreans were wearing life jackets at the time of the accident.