Prime Minister Yoshiro Mori apologized before the Diet on Wednesday for having invited a barrage of criticism by saying earlier this week that Japan is a "divine nation centering on the Emperor," but he refused to retract the remark.

"If (my remark) caused misunderstanding, it is regrettable and I would like to apologize," Mori told the Upper House plenary session, which had halted for two hours due to the row over his remark.

He said he never meant to say the Emperor is a god, stressing that the monarch is the symbol of the nation as stipulated in the Constitution.

Mori also stressed that the principles that sovereignty rests with the people and freedom of religion shall be firmly maintained in the future.

Mori made the controversial remark Monday at a gathering of like-minded pro-Shinto lawmakers.

Opposition parties claim Mori's remark runs counter to the nation's principle of democracy and the separation of politics and religion as stipulated in the Constitution.

Later in the day, four major opposition parties agreed to form a united front in a bid to jointly force Mori out of office over his remark.

The secretaries general of the Democratic Party of Japan, the Japanese Communist Party, the Liberal Party and the Social Democratic Party said they shared the view that Mori's speech did not constitute an apology but a poor excuse. They also charged that Mori lacks the qualifications to be prime minister.

The parties said they would pursue Mori's resignation through Diet deliberations and debate sessions, but added that they would not stage a boycott of Diet sessions, even if their demand is not met.

"(The remark) was (the vocalization of) an idea that seeps out from deep inside the prime minister. This can't be resolved just by asking for a withdrawal," DPJ leader Yukio Hatoyama said.

The remark attracted criticism even from the ruling bloc, especially from New Komeito, which is backed by the lay Buddhist group Soka Gakkai.

Sources said Mori called New Komeito chief Takenori Kanzaki early Wednesday to apologize.

During the Upper House plenary session, the prime minister maintained that he had just wanted to stress the importance of "moral education" for children amid a series of heinous crimes committed by teenagers. But he was not referring to any particular religion, he said.

Kanzaki said his party would accept Mori's explanation to the Diet. He added, however, that the prime minister, who is known to have made several other gaffes, should be more careful when speaking in public.

That view was echoed by Chief Cabinet Secretary Mikio Aoki, who told an afternoon news conference that he had already advised the prime minister to be more careful about his remarks.

"I will need (him) to fully bear in mind the occasions as well as his position when he speaks," Aoki said. He also stressed that Mori's refusal to retract the remark was a decision made by the prime minister himself.

Mori had told reporters as much earlier in the day.

"While I apologize for giving the wrong impression by not conveying my opinion properly, it does not mean I will rescind the remark," he said at his official residence in the morning.

Diet sources said the confusion surrounding the remark was exacerbated by Mori's lack of understanding of the gravity of the situation and the relatively shaky footing he has within his own administration.