Strong rip currents dragged 143 swimmers away from the shore at Haeundae Beach in Busan on Saturday, when sizzling heat sent 800,000 vacationers flocking to the popular holiday spot.

Rip currents can sweep swimmers far out into the ocean very quickly and are believed to form when the underwater slope is steep and the seabed warped, but the exact causes are unknown.

Maritime police at Haeundae said the riptides occurred at around 10:45 a.m., 12:20 and 12:40 p.m on Saturday between watchtowers No. 5 and 7. The first 66 swimmers were dragged 70 to 80 m out and required the help of lifeguards to get back to the beach.

Haeundae Beach in Busan is packed with holidaymakers on Sunday. /Newsis

Another 40 swimmers had to be rescued around midday. Maritime police eventually cordoned off the entire beach at 12:15 p.m. and did not allow swimmers back in until 3:30 p.m., but another 30 had to be rescued later in the day.

"There were no riptides spotted from June 1, when the beach opened, until the end of July," maritime police said. "But we had to rescue around 100 swimmers on Thursday and Friday due to riptides. Having to rescue 143 people in one day is quite unusual."

The Korea Meteorological Administration warned more riptides could occur on the southern coast on Monday.