The mass, which will see the pope beatify 124 early Korean martyrs, is the centrepiece of his five-day visit to South Korea, the first papal trip to Asia in 15 years.

Three hours before the event was scheduled to begin at 10:00am (0100 GMT), Gwanghwamun boulevard was already crammed with spectators for a one-kilometre (half-mile) stretch north of City Hall.

Some had begun arriving as early as 3:30am, and whiled away the time quietly reading their bibles in small groups.

The papal stage, topped with a giant cross stood at the top of the boulevard, backed by the giant tiled roof of the Joseon dynasty Gyeongbokgung Palace.

Several giant screens set up around the stage showed portraits of the martyrs to be beatified, as well as aerial shots of the huge crowd.

Only 200,000 people, most of them members of church groups who pre-registered for the event, were allowed to pass through metal detectors and ID checks to enter a 4.5km long security ring placed around the main plaza.

Others packed the surrounding side streets, relying on large plasma screens to follow the ceremony.

In the 18th and 19th centuries, unrepentant Catholics were generally paraded from Gwanghwamun southwest to Seosomun Gate where they were publicly executed.

Pope Francis was to begin the day at a martyrs' shrine at Seosomun and then make the journey of the condemned in reverse to Gwanghwamun.

Armed troops were guarding major bridges on the Han River into downtown Seoul, and many people had a lengthy walk to the venue, with public buses diverted and subway stations in the immediate vicinity shut down.

Those allowed inside the inner security cordon were banned from carrying any plastic containers, whistles, or sharp items including umbrellas.

Organisers had been concerned about the relatives of victims of April's Sewol ferry disaster, who have been camped out in Gwanghwamun for weeks to push their campaign for a full independent inquiry into the tragedy, which claimed 300 lives -- most of them schoolchildren.

In the end, 600 family members were invited to attend the mass, effectively incorporating the protest into the event.

On a small yellow tent pitched 150 meters in front of the papal altar, the relatives had written the message: "You love those suffering, Papa. Sewol families are here."

"I am a Buddhist but I think the Pope can help us," said Choi Keum-Bok, a construction worker who lost his son in the disaster.

The pope held a brief private audience with a group of Sewol survivors and family relatives on Friday, before holding a mass in the central city of Daejeon. (AFP)