Japanese PM Shinzo Abe is seeking a public mandate on his tougher diplomatic and defence policies to deal with escalating threats from North Korea (AP Photo/Shizuo Kambayashi)

Japanese prime minister Shinzo Abe took to the streets after calling a snap election, telling voters only he could protect them from the threat of North Korean missiles.

Mr Abe, of the Liberal Democratic Party, also t old a crowd in Tokyo's busy Shibuya district that only his party that can implement appropriate measures to deal with Japan's rapidly ageing and declining population.

"I must seek your support in order to overcome this national crisis," Mr Abe said.

The premier dissolved the lower house of parliament earlier on Thursday, calling an election on October 22.

Opposition politicians scrambled to regroup around Tokyo governor Yuriko Koike's new party to challenge Mr Abe's ruling party.

Mr Abe is widely seen as trying to reconsolidate his grip on power within his party, so he can extend the term of his premiership next year.

The dissolution of the more powerful of Japan's two-chamber parliament comes more than a year before required by law.

However, the ruling party faces a growing challenge from a new party launched by Ms Koike this week.

The Party of Hope has energised some voters, and is gaining renegade politicians from the main opposition party.

The speaker of the house, Tadamori Oshima, read the statement of dissolution.

Lower house members all stood up and chanted "banzai" three times in a dissolution ritual, then rushed out of the assembly hall.

Minutes after the dissolution, Mr Abe made a fiery speech to party members.

He said he is seeking a public mandate on his tougher diplomatic and defence policies to deal with escalating threats from North Korea and that his party members would have to relay his message to gain support from voters during the campaign.

"This election is about how we protect Japan, the people's lives and peaceful daily life," he said.

"The election is about the future of our children."

The Cabinet later approved an October 22 election for the 475-seat lower house.

The other chamber, the upper house, does not dissolve but is closed until parliament is reconvened after the election.

Support ratings for Mr Abe's government had plunged to below 30% in July following repeated parliamentary questions about allegations he helped his friend obtain approval to open a veterinary college.

Recent media polls show the support ratings recovering to around 50%, helped by parliament's recess and a Cabinet reshuffle in August which removed the defence minister and several other unpopular faces.

It was a significant turnaround from July, when the party suffered a devastating loss in a Tokyo city assembly election to maverick Ms Koike's new regional party.

According to the Mainichi newspaper poll taken this week and published on Thursday, Mr Abe's party was a top choice among voters at 29%, followed by Ms Koike's party, which came in second at 18% and moving ahead of other opposition parties such as the Democratic Party at 8%.

Analysts believe Mr Abe's ruling party will retain a majority, though some seats may be pulled by Ms Koike's party.

Voters may have difficulty figuring out clear differences in policies between the two parties, they say.

The two leaders share nationalistic views and they want to change the war-renouncing Constitution.

The main opposition Democratic Party, which held power in 2009-2012, has lost ground largely due to internal disagreements, and is now falling apart, and many members have defected to Ms Koike's party.

Democratic leader Seiji Maehara has proposed allowing the remaining members to make their own decision whether to leave or stay, setting the stage for a possible merger with Ms Koike's party.

AP

PA Media