TEHRAN (Reuters) - A newspaper editor seen as close to Iran's top authority said Saturday defeated election candidate Mirhossein Mousavi and a former pro-reform president had committed "terrible crimes" which should be tried in court.

In a commentary published in his hardline Kayhan daily, editor-in-chief Hossein Shariatmadari suggested that Mousavi and his supporters in last month's disputed election had acted on the instructions of the United States, Iran's arch-foe.

The June 12 poll stirred the most striking display of internal dissent in Iran since the 1979 Islamic revolution and strained ties with the West. At least 20 people died in post-election violence last month.

The authorities have portrayed mass pro-Mousavi protests, which erupted after official results showed President Mahmoud Ahmadinejad had been re-elected by a landslide, as the work of local subversives and foreign powers.

"Documents and undeniable evidence show that this mission was directed from the outside," Shariatmadari, who is close to Supreme Leader Ayatollah Ali Khamenei, wrote in a commentary.

"All they did and said was in line with the instructions announced by American officials in the past," he added.

He also singled out former President Mohammad Khatami, a leading reformist who backed Mousavi in the election campaign.

"An open court, in front of the people's eyes, must deal with the all the terrible crimes and clear betrayal committed by the main elements behind the recent unrest, including Mousavi and Khatami," Shariatmadari wrote.

His comments were a clear sign of mounting hardline pressure on Mousavi and his allies, who say the election was rigged in favor of the anti-Western incumbent.

"There were some reasons why the United States, Israel, the European Union, all anti-revolutionary groups ... supported Mousavi and his group and spent all their political and media forces to support this group," Shariatmadari wrote.

Another hardline newspaper, Javan, said 100 members of parliament had signed a letter to the judiciary calling for the leaders of "post-election riots" to face trial.

The authorities reject opposition charges of vote rigging and say the vote was Iran's "healthiest" since the revolution.

They blame Mousavi, a moderate former prime minister, for the bloodshed after the election. Mousavi rejects the charge.

Although hardliners have regained the initiative since security forces quelled the protests, Mousavi and another losing candidate, pro-reform cleric Mehdi Karoubi, have not yielded.

They again denounced the election result Wednesday and said Ahmadinejad's next cabinet would be illegitimate.

(Writing by Fredrik Dahl; editing by Alistair Lyon)