Mourners streamed into a temple in Tokyo to pay their respects to Japan's slain former premier Shinzo Abe on Monday, as his assassination overshadowed an election win for the ruling party he had dominated.

Current Prime Minister Fumio Kishida, who has the chance to cement his own power following Sunday's election gains, and U.S. Treasury Secretary Janet Yellen were among the hundreds at Abe's wake, three days after he was shot at an election rally.

A private funeral for Abe, who resigned in 2020 and was Japan's longest-serving premier, is scheduled for Tuesday.

"There is a profound sense of sorrow at his loss," Yellen told reporters outside the temple, where she placed incense in Abe's honour and greeted his family.

"Prime Minister Abe was a visionary leader and he strengthened Japan...I know that his legacy will live on," she added.

Abe's shooting shocked a nation where political violence and gun crime is rare.

The suspected killer, arrested at the scene and identified by police as 41-year-old Tetsuya Yamagami, believed Abe had promoted a religious group to which his mother made a "huge donation", Kyodo news agency has said, citing investigators.

The Unification Church, a controversial group known for its mass weddings and devoted following, said on Monday the suspect's mother was one of its members.

Neither Abe nor Yamagami were members of the church, said Tomihiro Tanaka, president of its Japan branch, adding that it would cooperate with police if asked to do so.

Reuters was not immediately able to contact Yamagami's mother and could not determine whether she belonged to any other religious organisations.

Sombre victory

In elections held on Sunday, the Liberal Democratic Party (LDP) and its ruling coalition partner extended their majority in the upper house of parliament. With a majority already in place in the lower house, what would have been a celebratory mood at LDP headquarters in usual circumstances turned sombre.

A moment of silence for Abe was offered in his memory, and Kishida's face remained grim as he pinned rosettes next to winning candidates' names on a board in a symbol of their victory.

Abe's death has drawn condolences from leaders around the world, from Britain's Queen Elizabeth to Chinese Premier Xi Jinping.

U.S. Secretary of State Antony Blinken met with Kishida during a brief stopover on Monday to offer messages of support on behalf of President Joe Biden.

Vice President William Lai became Taiwan's most senior official to visit Japan in five decades as he made a private trip to Tokyo to pay his respects.

At the wake, a line of black sedan cars, including several with diplomatic plates, dropped off dignitaries and family at Tokyo's Zojoji temple, where the ex-premier's body lay.

Dressed in black, some of those gathered mopped their brows as they queued beneath the steps leading to the temple in the sultry evening air.

A part of the temple was also open to members of the public who crowded in to lay flowers. "I feel so sad that a prime minister who dedicated himself for Japan died this way," said Naoya Okamoto, a 28-year old who works in construction.

"He was the prime minister who demonstrated to the world a strong Japan once again."