SEOUL - South Korea dialed down the volume on Thursday as hundreds of thousands of students sat a crucial national university entrance exam, with authorities taking extraordinary measures to minimize possible distractions.

The college entrance test is the culmination of South Korea's highly demanding school system, and in an ultra-competitive society it plays a large part in defining students' adult lives, holding the key to top universities, elevated social status, good jobs, and even marriage prospects.

This year nearly 595,000 students were sitting the grueling exam, which stretches over nine hours, according to the education ministry.

South Korean President Moon Jae-in, who was in Singapore for regional summits, posted a good luck message to the students on his Facebook page.

The intense effort they had put in over years of study was coming to fruition, he said. "Believe in it and you will be able to show your full competence."

Extraordinary measures are taken nationwide to remove anything that could disturb the test-takers.

Public offices, major businesses and the stock market opened an hour later than usual to help ease traffic and ensure students arrived on time for the exam, which began nationwide at 8:40 am (2340 GMT).

Any students stuck in traffic could get police cars and motorbikes to rush them to the exam centers.

All takeoffs and landings at South Korean airports are suspended for 25 minutes to coincide with an English listening test, and all airborne planes must maintain an altitude higher than 3,000 meters (10,000 feet).

The Transport Ministry said 134 flights had to be rescheduled because of the exam.

Electronics are strictly forbidden and students cannot leave school premises until the test ends to reduce the chances of cheating.

But they will be allowed to wear masks during the exam this year, the education ministry said, with fine dust pollution levels persisting at "bad" on the peninsula.

The results of the daunting exam will be released on December 5.