Two former prime ministers who have become anti-nuclear campaigners in the wake of the 2011 Fukushima disaster joined a rock festival in Tokyo that brought together musicians opposed to atomic energy.

"We must create a country where nuclear power generation is zero," Junichiro Koizumi shouted from the stage Monday in front of about 2,500 people. "Let's develop our country without nuclear power."

Koizumi, prime minister from 2001 to 2006, also told reporters later that an "unexpected event could happen anytime" at nuclear power plants in Japan, touching on the recent volcanic eruption of Mount Ontake, which occurred without warning and is believed to have killed at least 36 hikers.

"In Japan, earthquakes, tsunami and volcanic eruptions could occur in various locations," he said. "It's a country that's not allowed to have nuclear power plants."

Morihiro Hosokawa, who served as prime minister from 1993 to 1994, said he felt hopeful about the anti-nuclear campaign when he saw the eyes of youths at the festival "twinkling."

The rock concert, organized by renowned musician and composer Ryuichi Sakamoto, has been held annually since 2012.

Koizumi and Hosokawa said they had no plans to become involved in the Oct. 26 gubernatorial election in Fukushima Prefecture, which is home to Tokyo Electric Power Co.'s crippled Fukushima No. 1 nuclear plant.

"People recognize (the importance of) breaking away from nuclear power. It will not be an issue that will be contested (in the election)," Hosokawa said.

All nuclear power stations in Japan are currently offline, but the Abe administration is pushing for the resumption of reactors that have cleared the safety requirements introduced in the wake of the Fukushima triple meltdown.