Up to one million people are expected to converge on the capital's central Gwanghwamun Square for the mass, which will mark the religious centrepiece of the pope's five-day visit to South Korea.
The most prominent among those to be beatified is an 18th century nobleman, Paul Yun Ji-Chung, who became Korea's first Catholic martyr when he was executed in 1791 after clashing with Confucian officials.
According to the Church, around 10,000 Koreans were martyred in the first 100 years after Catholicism was introduced to the peninsula in 1784.
Uniquely, the religion was not brought in and spread by foreign missionaries, but by Korean scholars who had come across Catholic teachings in China and shared them on their return with family and friends.
It survived, as a largely illegal community, with virtually no formal missionary priests until clergy from France arrived more than 50 years later.
- Clash with Confucianism -
Born to a renowned noble family in what is now the southwestern county of Geumsan, Yun was introduced to Catholicism by his cousin, Kwon Sang-Yeon, and was baptised in 1787 by Korea's first Catholic convert, Peter Yi Seung-Hun.
Yun converted his brother and also his mother, who requested Catholic rites for her funeral rather than the accepted Confucian ritual.
Yun not only acceded to her wishes but also smashed his family's wooden ancestral tablets -- the focus of Confucian worship.
News of his actions triggered outrage and eventually spread to the royal court, which ordered a local magistrate of Jinsan to arrest Yun and Kwon.
Despite brutal interrogation, the two refused to disavow their faith or reveal the names of other Catholic followers.
"Though the bodies of Yun Ji-Chung and Kwon Sang-Yeon were covered all over with blood, they did not even groan," the local governor wrote in a report to the royal court.
"They refused to renounce their faith .... saying it was a great honor to die for God under the blade of a knife."
Their eventual executions in 1791 -- they were both beheaded -- were followed by the first in a serious of bloody Catholic pogroms.
In Seoul, martyrs were generally paraded from Gwanghwamun southwest to Seosomun Gate where they were publicly executed.
On Saturday Pope Francis will make the journey in reverse before conducting the beatification mass in Gwanghwamun plaza.
"The papal journey symbolises a reversal of the condemnation of the martyrs and their rehabilitation," said Joseph Lee Joon-Seong, the priest who looks after the martyrs' shrine at Seosomun.
The 124 who will be beatified include one Chinese martyr, Zhou Wenmo, a priest who was executed in 1801 after surrendering to spare his lay followers further torture.
Security will be very tight for the mass, with around 30,000 police on duty and snipers deployed on rooftops overlooking the main plaza.
Although police say they expect a million people to turn up, only 200,000 who pre-registered will be allowed inside the central ceremonial area, which will be ringed by a 4.5 kilometre long fence.