Shinzo Abe has declared victory in Japan's national elections with polls indicating his party has won a majority of seats in the upper house of Parliament, and is now set to become the country's longest-serving prime minister, CNN reports.

The ruling Liberal Democratic Party (LDP) won 71 of the 124 seats in the 245-member House of Councillors, according to public broadcaster NHK, falling short of the super majority required to complete its longstanding goal of changing the county's pacifist constitution.

The Constitutional Democratic Party (CDP), the main opposition body, won 53 seats, according to NHK.

"I would like to express my gratitude to the voters," said Abe in a press conference, Monday afternoon. "This is a choice about the political stability and chaos. We asked the voters to choose between these two options and many people listened to our speeches on the streets."

Abe laid out a variety of challenges his party intends to tackle during the next term, including social security reforms, free education, strengthening the economy, and addressing the aging population and shrinking workforce.

Sunday's result leaves Abe with a clear majority in both houses of Parliament, following a landslide win in the lower house in 2017. However, Abe failed to achieve the two-thirds majority needed to amend the constitution -- a signature goal for which Abe has set a 2020 deadline.

"In this election, the constitutional amendment was also a big issue — it will be finally determined by the national referendum," Abe said on Monday, calling the two-thirds requirement "a very tall order."

"The people of Japan will have a final say on this matter."