Three former executives of Tokyo Electric Power (Tepco) on Friday pleaded not guilty to professional negligence leading to the 2011 Fukushima nuclear disaster, though an analyst said there is little chance they will be convicted.

Despite the not guilty pleas, former Tepco chairman Tsunehisa Katsumata, 77, and former executives Sakae Muto, 67, and Ichiro Takekuro, 71, apologised during the hearing at the Tokyo District Court for causing trouble to the victims and society, according to a pool report for foreign journalists.

They are the first individuals to face criminal charges for the Fukushima Daiichi nuclear disaster but a high bar for proof may prevent a conviction. Prosecutors earlier declined to bring charges but a civilian judiciary panel twice voted to indict the executives, overruling the prosecutors the second time.

"I apologise for the tremendous trouble to the residents in the area and around the country because of the serious accident that caused the release of radioactive materials," Katsumata said, adding he could not have anticipated the risk of high tsunami.

Lawyers acting as prosecutors said the three executives had access to data and studies anticipating the risk of a tsunami exceeding 10 metres height in the area that could trigger power loss and cause a nuclear accident, according to the pool report.

However, lawyers for the defendants said that such tsunami estimates were not well established, and even experts had divisive views.

The defendants' lawyers were not immediately available to the media, a court official said.

Citizen judiciary panels, selected by lottery, are a rarely used feature of Japan's legal system introduced after World War Two to curb bureaucratic overreach.

In 2015, Tokyo prosecutors twice declined to bring charges against the three, citing insufficient evidence. But the 11 unidentified citizens on the panel voted for a second time in July that year to indict them.

The panel found the former executives had failed to take countermeasures to strengthen the Fukushima plant despite foreseeing the dangers it faced from tsunamis, according to a copy of the 31-page ruling seen by Reuters.

Indictments brought by the panels have a low conviction rate. One review of eight types of these cases by the Eiko Sogo Law Office found just one, equal to a 17 percent conviction rate, compared with an overall rate of 98 percent in Japan.

"I think the likelihood of a conviction is exceptionally low, particularly a conviction that survives all the way to the Supreme Court," said Colin Jones, a professor at the Doshisha Law School in Kyoto.

"Tokyo District Court is probably one of the most institutionally conservative courts in the system, so it is a court that is very likely to be government/large company-friendly in this sort of case," he added.

The Fukushima plant, located 220 km (130 miles) northeast of Tokyo, was rocked by a magnitude 9.0 earthquake and subsequent tsunami in March 2011, sparking three reactor meltdowns. More than 160,000 residents fled nearby towns and radiation contaminated water, food and air in the world's worst nuclear disaster since Chernobyl in 1986.

The current trial is expected to last more than a year, the Asahi newspaper said.