SEOUL (AFP) - South Korea went into "hush" mode on Thursday, as nearly 650,000 students sat for the annual college entrance exam that will play a large part in defining their adult lives in an ultra-competitive society.

Preparation for the crucial exam starts from primary school, and so does the relentless pressure which has been blamed for everything from early burnout to teenage depression and suicide.

Success means a secured place in one of South Korea's elite universities and is seen as the key to future careers as well as marriage prospects.

With so much riding on the outcome, the day of the test - simultaneously in 1,257 centres nationwide - sees the entire country switch to silent running.

The transportation ministry bans all airport landings and departures for a 40-minute period to coincide with the main language listening test. The military also reschedules airforce drills and live-firing exercises, while traffic is barred within a 200-meter radius of the test centres.

Public offices and major businesses, as well as the stock markets, opened an hour later than usual to help keep the roads relatively clear and ensure the students arrived on time for the exam which began at 8.40am (7.40am Singapore time).

Anyone who is stuck in traffic could dial the emergency number 112, and request help from police cars and motorbikes on standby to rush them to the centres.

At Seoul's Pungmoon Girls' High School, junior students, huddled together in the -3 deg C cold, held good-luck banners and shouted encouragement as their seniors entered the exam room.

For the equally-stressed parents, for whom their child's result will partly be seen as a mark of their parental aptitude and devotion, there was little left to do after a final hug at the school gates.

Many immediately made their way to nearby churches and temples in search of some divine intervention.