The annexation completed a years-long process by the Japanese effectively making the Korean peninsula a colony. In Seoul people still harbor strong resentment about their one-time colonial masters a century after the event.
A brass band played martial tunes under a tent in pouring rain at a civic event marking the date 100 years ago that Korea formally came under Japanese rule.
Many Asian historians say that was the result of an unfair treaty forced upon the Korean kingdom.
It turned out to be a brutal experience for Koreans. Their language and culture were suppressed. Millions of Koreans found themselves working for the Japanese in deplorable conditions, some effectively slave laborers.
The two rival Koreas - which have no diplomatic relations with each other - have now been independent for 65 years. But many on both sides of the border say the wounds have not healed.
North Korea is making a fresh demand for reparations, noting the recent apology by Japanese Prime Minister Naoto Kan was only directed at South Korea.
In Seoul, at the ceremony marking a century since annexation, the vice chairman of the Korea Liberation Association, Nam Man Woo, says he does not believe Japan has done enough to repair the relationship with his country.
Nam says Japanese prime ministers and emperors have apologized several times but they were merely expressing words.
South Korea, lifted a long-standing ban on "Japanese cultural imports" six years ago. That has created a new generation of South Koreans who are enthusiastic consumers of Japanese pop music, literature and movies. But the lessons of history create mixed feelings for students such as 12-year-old Baek Ju Hee.
The sixth-grade girl says she thought Japanese were nice because figure skaters Mao Asada of Japan and South Korea's Olympic gold medalist Kim Yu Na are friendly rivals. But recently, after learning in school about what Japan did to Korea, she now hates Japan.
While apologizing for the annexation, Japan's government has not acknowledged the treaty was illegal - as many on the Korean peninsula contend.
Japan's foreign minister, Katsuya Okada, last week told reporters in Tokyo the matter should be resolved among academics as the issue likely will never be resolved by government officials.