Russia's Central Election Commission has approved the opposition liberal party Yabloko to contest September's parliamentary elections, making it the only party openly campaigning against the country's war in Ukraine.

The commission said it had unanimously approved Yabloko's list of 269 candidates for the State Duma elections, scheduled for September 18-20.

Yabloko members have long been outspoken critics of Russia's full-scale invasion of Ukraine, and many have faced what supporters describe as political persecution and arbitrary prosecution.

The party is campaigning under the slogan "For Peace and Freedom."

"This is an important moment for Russia," Yabloko chairman Nikolai Rybakov said on Wednesday. "For the first time in five years, citizens have the opportunity to vote peacefully and safely for the future, for peace, for freedom and for a Russia without fear."

Rybakov himself had been barred from running. The party's electoral list is headed by Yaroslav Shcherbakov, who said voters now had an alternative while acknowledging the challenges ahead.

"There are no illusions, but there is hope," he said.

In previous elections, many Yabloko members were barred from standing. Central Election Commission chair Ella Pamfilova said Russia had "two poles" - those who support the war and those who oppose it - and that it was up to voters to decide between them.

Nine parties have now been registered to contest the State Duma elections, although 11 parties had sought to field candidates.

Rybakov said Yabloko would campaign for a ceasefire in Ukraine.

"We will do everything to bring peace, so that no more people die, and our country has a future," he said.

Political commentators in Moscow said the Kremlin may have allowed Yabloko to participate because weak polling for the ruling United Russia party had created a need for a controlled outlet for dissatisfied voters.

At the same time, analysts said Yabloko poses little political risk to the Kremlin because many of its best-known figures were barred from standing and the party remains electorally weak.

Opponents of the war continue to face intense pressure in Russia.

Earlier this week, opposition politician Boris Nadezhdin, who also advocates ending Russia's invasion of Ukraine, withdrew from the election after a brief detention. He said he also wanted to avoid putting his supporters at risk.