South Korea's first female president, Park Geun-hye, has taken the oath of office at a ceremony in the capital, Seoul.
Ms Park promised a tough stance on national security and an era of economic revival as she was sworn in as South Korea's president.
Ms Park, who defeated liberal rival Moon Jae-in in December's general election, took the oath of office in front of tens of thousands of people.
North Korea's recent nuclear test posed a "challenge to the survival" of the Korean people, she said.
Trust-building was needed to tackle the "extremely serious" security situation.
Ms Park, the first woman to lead South Korea, succeeds President Lee Myung-bak, of the same Saenuri Party, who stepped down as the law required after a five-year term.
The BBC reports the 61-year-old is the daughter of former military strongman Park Chung-hee, who ruled South Korea for almost two decades.
She takes office amid high tensions on the Korean peninsula in the wake of a North Korean nuclear test, on 12 February.
