Five lawmakers filed candidacies Friday to lead Japan's opposition party and possibly become prime minister following the next elections, calling for efforts to boost the country's economy and shore up its defense.

The opposition Liberal Democratic Party has ruled Japan for most of the time since World War II. Analysts are forecasting it likely will win the largest number of parliamentary seats in elections yet to be called, but it would likely need a coalition to gain majority control.

The party has been gaining in polls due to voter dissatisfaction with a perceived lack of leadership by the ruling Democratic Party of Japan, whose popularity has languished as the economic recovery has stalled and the government has struggled with the aftermath of last year's earthquake, tsunami and nuclear disasters.

In a televised debate Friday, the candidates called for rebuilding the economy and strengthening Japan's defense at a time of deepening tensions with China.

The sense of crisis sharpened after Chinese surveillance ships entered Japanese waters early Friday near disputed islands in the East China Sea, adding to tensions between the Asian powers.

The candidates are mostly political veterans, including former Prime Minister Shinzo Abe, former Foreign Minister Nobutake Machimura, former Defense Minister Shigeru Ishiba, LDP Secretary General Nobuteru Ishihara and former economic minister Yoshimasa Hayashi.

"Our party has done its best, and that's why it held power for so long," Ishiba said. "We the LDP must once more win the confidence of the public."

How to manage relations with China is one of Japan's biggest challenges, Hayashi said.

"We must ensure the peace and security of the region," he said. "The balance of power may change in the next five years. We must ensure that the changes are to our country's benefit."

Ishihara is the son of Tokyo Mayor Shintaro Ishihara, who sparked the current round of friction with Beijing over the disputed islands — known as Senkaku in Japan and Diaoyu in China — by proposing the city's government buy them from their private owners. Instead, the central government purchased the islands this week, sparking an angry response from Beijing.

"I believe we may be headed for our worst frictions with China since we re-established diplomatic relations," Machimura said.

Prime Minister Yoshihiko Noda, who has been in office just over a year, has said he would call general elections soon, bringing on yet another possible change of leadership at a time when Japan is confronting myriad economic and strategic challenges. Media reports speculate the elections could be held between November and January.

Noda is Japan's sixth prime minister in six years. He is expected to win against three challengers for leadership of the Democratic Party in a vote Sept. 21.