A candidate backed by opposition parties won Sunday's mayoral election in Sendai, dealing another blow to the administration of Prime Minister Shinzo Abe, who leads the LDP.

The loss by an LDP-supported candidate apparently reflected the growing distrust of the Abe administration facing a host of problems such as alleged favoritism toward a school operator headed by a longtime friend of Abe.

"The defeat in the biggest city in the Tohoku (northeastern Japan region) is shocking. The LDP may lose support further," a young party member said.

"There was apparently criticism of the administration," said Tetsuo Saito, election chief at Komeito, the LDP's ruling coalition partner. A humble approach is important, he said.

The outcome of the election is "undoubtedly a serious blow to the arrogant Abe administration," said Jun Azumi, deputy president of the largest opposition Democratic Party. "We're now aware that voters will support if opposition parties team up," he said.

"There was a clear display of anger at the administration," said Akira Koike, head of the secretariat at the Japanese Communist Party.

Abe aims to reshuffle his cabinet and the LDP's executive team on Aug. 3 in an effort to overcome his current difficulties. But he also faces intense pressure over sharp declines in his cabinet's public approval ratings and the Defense Ministry's alleged cover-up of daily reports by Self-Defense Forces troops deployed in South Sudan on a UN peacekeeping mission.

The next test for the administration will come on Sunday when a mayoral election is scheduled to take place in Yokohama, south of Tokyo, which includes the constituency for Chief Cabinet Secretary Yoshihide Suga, a key player in the Abe cabinet.

On national politics, a House of Representatives by-election will be held in a constituency in the western prefecture of Ehime on Oct. 22.