* PM Abe seeks mandate on social security, N.Korea stance
* Abe seen taking advantage of opposition disarray
* He says target is simple majority in lower house
* Abe orders 2 trln yen package for child care, education (Adds PM quotes throughout)
TOKYO, Sept 25 (Reuters) - Japanese Prime Minister Shinzo Abe said he would dissolve parliament's lower house on Thursday for a snap election, seeking a mandate to stick to his tough stance towards a volatile North Korea and rebalance the social security system.
Abe, in power for five years, had been expected to call the election for next month to take advantage of improved support and disarray in the opposition camp.
"I'll demonstrate strong leadership and stand at the forefront to face a national crisis," Abe told reporters, mentioning Japan's fast-ageing population and North Korea.
"This is my responsibility as leader and my mission as prime minister."
Natsuo Yamaguchi, the head of Abe's junior coalition partner the Komeito party, said he understood the election would be on Oct. 22.
Abe said he would redirect some revenue from a planned sales tax hike in 2019 to child care and education rather than paying back public debt, although he added he would not abandon fiscal reform. Rebalancing the spending would offset the potential negative effect on consumption from the tax rise, he said.
"We will turn Japan's social security system into one that responds to all generations by boldly diverting policy resources to resolve the two major concerns -- child rearing and (elderly) nursing care that working generations confront," he said.
Abe rejected criticism that holding an election now would create a political vacuum at a time of rising tension over North Korea's missile and nuclear arms programme. Pyongyang has fired ballistic missiles over Japan twice in the last month and conducted its sixth and biggest nuclear test on Sept. 3
"We must not give in to North Korea's threats. By gaining a mandate from the people with this election, I will forge ahead with strong diplomacy," Abe said, adding that now was the time to put more pressure on Pyongyang, not open dialogue.
Abe, whose ratings have risen to around 50 percent from around 30 percent in July, is gambling his ruling bloc can keep its lower house majority even if it loses the two-thirds "super majority" needed to achieve his long-held goal of revising the post-war pacifist constitution to clarify the military's role.
He said his goal was for his coalition to retain a majority in the chamber.
A weekend survey by the Nikkei business daily showed 44 percent of voters planned to vote for Abe's Liberal Democratic Party (LDP) versus 8 percent for the main opposition Democratic Party and another 8 percent for a new party launched by popular Tokyo Governor Yuriko Koike.
The Nikkei poll was more positive for Abe's prospects than a Kyodo news agency survey that showed his LDP garnering 27.7 percent support, with 42.2 percent undecided.
Abe's image as a strong leader has bolstered his ratings amid the North Korea crisis and overshadowed opposition criticism of the premier for suspected cronyism scandals that eroded his support earlier this year.
Given the results seen in other major developed countries, however, some political analysts are not ruling out the unexpected.
"Abe's big gamble could yield a big surprise," veteran independent political analysts Minoru Morita said.
The prime minister had been expected to face a grilling over the cronyism scandals during a session of parliament from Thursday and opposition party officials saw the move as ploy to avoid difficult questions.
Abe on Monday asked his cabinet to compile a 2 trillion yen ($17.8 billion) economic package by year-end to focus on child care, education and encouraging corporate investment.
He told the news conference that diverting sales tax revenue would make it hard to meet a target of putting Japan's budget balance in the black by the year beginning April 2020, but added he would not give up on efforts to repair Japan's finances.
The main opposition Democratic Party is struggling with single-digit ratings and much depends on whether it can cooperate with liberal opposition groups.
On Monday, just hours before Abe's election announcement, Tokyo Governor Koike said she would lead a new conservative, reform-minded "Party of Hope" to offer voters an alternative to the LDP.
"Our ideal is to proceed free of special interests," Koike, a former LDP member, told a news conference.
An LDP internal survey showed seats held by the LDP and its coalition partner Komeito could fall to 280 from the 323 they now hold, the Nikkei reported on Saturday. Reforms adopted last year will cut the number of lower house seats to 465 from 475. ($1=112.3400 yen) (Additional reporting by Takashi Umekawa, Stanley White, Tim Kelly, Elaine Lies and Kiyoshi Takenaka; Editing by Michael Perry and Clarence Fernandez)