By Linda Sieg, Nathaniel Taplin and Ben Blanchard

TOKYO/SHANGHAI (Reuters) - Japanese Prime Minister Shinzo Abe, seeking to allay concern he would divert attention from fixing a fragile economy to revising a pacifist constitution after a big election win, said on Monday changing the charter would not be easy.

Abe's coalition and allies won two-thirds of the seats in parliament's upper house in a Sunday election.

That victory, with the ruling bloc's two-thirds majority in the lower house, opens the door to revising the constitution for the first time since its adoption after Japan's defeat in World War Two.

But a push to ease the charter's constraints on the military operating overseas is bound to be opposed by China, where memories of Japan's past militarism arouse anger.

China's official agency warned that the victory for Abe's party posed a danger to regional stability and a Foreign Ministry spokesman said it raised concern.

Abe said revising the constitution was his Liberal Democratic Party's (LDP) cherished goal, but forging agreement on changes in the diverse pro-revision camp would not be easy.

Revisions require a two-thirds majority in both houses of parliament and a majority of votes in a public referendum.

"To realize revision of the constitution is my duty as LDP president," Abe told a news conference.

"But it is not that easy, so I hope debate will deepen steadily."

China's Xinhua news agency said the prospect was alarming.

"With Japan's pacifist constitution at serious stake and Abe's power expanding, it is alarming both for Japan's Asian neighbors, as well as for Japan itself," Xinhua said.

"Japan's militarization will serve to benefit neither side."

While Xinhua commentaries are not government statements, they often reflect official Chinese thinking.

Chinese Foreign Ministry spokesman Lu Kang said it was totally understandable that countries in Asia like China were concerned about Japan's "political direction".

"We have said again and again we hope Japan earnestly learns the lessons of history and pays attention to the concerns of its Asian neighbors and the international community," Lu told a daily news briefing.

China hopes to see a Japan that is committed to peaceful development and speaks and acts cautiously on military and security matters, he added.


Experts agreed that building agreement in Japan on amending the constitution for the first time would be tough.

"It's the first time to have two-thirds in both houses of parliament, but you can't find any issue on which the two-thirds can agree," said Gerry Curtis, professor emeritus at New York's Columbia University.

Some in financial markets worry focus on the constitution will distract attention from the economy, but Abe promised to craft a large stimulus package.

"He must boost support to advance revision. So for constitutional change as well, he will probably come up with a large-scale economic package," said Daiji Aoki, senior economist at UBS Securities Japan.

Doubts about Abe's policies persist even though his ruling bloc won big in terms of seats. Many voters felt they had no other option, given memories of the main opposition Democratic Party's rocky 2009-2012 rule.

Surveys show many voters are wary of changing the constitution's war-renouncing Article 9, which advocates see as the source of Japan's post-war peace and democracy.

Conservatives see it as a symbol of humiliating defeat.

If taken literally, Article 9 bans the maintenance of armed forces. Successive governments have interpreted it to allow a military for self-defense, a concept Abe last year stretched to allow Japan's military to aid friendly nations under attack.

But convincing the Komeito party, the dovish junior partner in Abe's coalition, to agree would be challenging. The pro-revision camp might therefore tackle another amendment first.

One possibility would be to introduce a clause giving the government more powers in a national emergency, but critics say that would endanger civil rights.

Another option, floated by the Komeito, would be to add an environmental protection clause to the constitution. That less contentious step would nonetheless break the taboo on revision.

(Additional reporting by Elaine Lies, Kiyoshi Takenaka and Daiki Iga in Tokyo, and Ben Blanchard in Beijing; Editing by Paul Tait, Robert Birsel)