Ansan (South Korea) (AFP) - A century after mass protests against Japanese colonial rule in Korea the issue of those who collaborated with Tokyo -- many of whom later become part of the South Korean elite -- remains hidden in the shadows.

When the Seoul government signed a 1910 treaty handing sovereignty over the peninsula to Japan, their new overlords awarded 76 key politicians and officials Japanese noble titles and pensions worth millions.

Over the next 35 years hundreds of thousands of Koreans worked for colonial authorities as civil servants, soldiers, teachers or police.

And according to historians hundreds of thousands more were forcibly recruited as frontline troops, slave workers and wartime sex slaves.

A few thousand others went into exile in China to fight Japanese forces.

The independence struggle is at the heart of Korean national identity in both North and South, but eight in 10 South Koreans believe their country has never properly come to terms with the issue of collaboration, according to a government study released for last week's 100th anniversary of the March 1 Independence Movement.

Mass protests against Japanese rule began that day in 1919, only to be forcibly put down, with 7,500 killed within two months and 46,000 arrested according to Seoul's national archives.

In a commemorative speech President Moon Jae-in said "wiping out the vestiges of pro-Japanese collaborators" was a "long-overdue undertaking".

But it is an intensely political issue, with collaborators generally seen as right-wing and Moon under pressure from conservatives looking to paint him as a Northern sympathiser. 

'Liberated homeland'

The US atomic bombings of Hiroshima and Nagasaki forced Tokyo's World War II surrender and ended colonial rule -- only for the victors to divide the peninsula.

In the North, Kim Il Sung's Moscow-backed regime executed Japanese collaborators en masse.

In the South, the US-oriented administration of Syngman Rhee recruited many colonial-era officers and officials into its ranks to exploit their expertise and experience.

"Even in the liberated homeland, those who used to serve as police officers during Japanese colonial rule painted independence activists as Reds and tortured them," Moon said.

Historical resentment sours relations with Tokyo to this day and prevents South Korea making a proper reckoning with its past, says Lee Young-hun, a controversial former Seoul National University economics professor.

"Those who are labelled as Japanese collaborators are Koreans who actively embraced modernism," Lee, who is derided by some South Korean media as a "colonialist", told AFP. 

National anthem

Among those who went into exile was Shin Young-shin's great-grandfather, a Korean general imprisoned and tortured by Japanese-backed troops. 

Both her parents took part in the campaign, but struggled to feed their family when they returned to the South in the late 1940s.

"My parents got nothing -- not even a penny from the government -- for their activism while they were alive," Shin, 71, told AFP in her small basement flat in Ansan, south of Seoul.

According to local government data almost three-quarters of independence activists' descendants in Seoul make less than two million won (US$1,800) a month.

But many descendants of collaborators -- defined under South Korean law as those who received titles under Japanese rule, arrested or killed independence fighters -- have prospered.

One of those ennobled in 1910, and included on a list of 1,005 collaborators Seoul issued 10 years ago, was Song Byung-jun. 

His son led the forces that jailed Shin's great-grandfather, and his grandson became the first director of Seoul's central bank.

Some of the South's biggest conglomerates were founded during the colonial period. High-profile executives, including Hyundai Group's chairwoman Hyun Jeong-eun, have sued to remove ancestors' names from the collaborators list.

Park Chung-hee was an officer in the Japanese forces before ruling the South as a military-backed dictator for 18 years until his 1979 assassination -- and his daughter went on to be elected president in 2012.

Park was not officially classed as a collaborator but is on a longer list of 4,389 names collated by the Center for Historical Truth and Justice (CHTJ) campaign group, which includes prominent cultural figures such as Ahn Eak-tai, composer of the South Korean national anthem.

"There is a popular Korean saying that translates to: 'Those who fought for independence have made three generations of their descendants suffer. Those who collaborated with the Japanese have made three generations of their descendents prosper'," said CHTJ researcher Lee Yong-chang.

Shin -- whose mother was posthumously awarded the Order of Merit for National Foundation -- will never come to terms with the contrast.

"I have prayed so that I could love my enemies," she said. "But I cannot possibly love the Japanese collaborators."