TBILISI (Reuters) - Georgia’s failure to pass a planned electoral reform is disappointing, a European rights body and the U.S. embassy said on Friday, urging lawmakers in the former Soviet state to approve the measure in time for next year’s election.

The change is one of the demands made at protests that have taken place in the capital Tbilisi since June and the ruling Georgian Dream Party had appeared ready to support it.

But parliament failed to pass the amendments on Thursday, triggering protests outside the building by thousands of opposition and civil activists.

The switch to full proportional representation from the roughly half at present was scheduled to happen in 2024, but the opposition demanded it be brought forward, saying the system unfairly favors Georgian Dream, which has ruled since 2012.

The party is pro-Western but opponents say it is too soft on Moscow. Its popularity has sagged since security forces clashed with protesters at an anti-Kremlin rally in June.

Parliament began discussing the amendments on Wednesday but some ruling party lawmakers opposed the changes, apparently fearing that proportional voting would cost them their seats.

“The failure of the amendments to pass is incomprehensible,” Titus Corlatean and Claude Kern, rapporteurs of the Parliamentary Assembly of the Council of Europe (PACE), said in a statement. “This is a step backwards.”

PACE is the parliamentary arm of the 47-nation Council of Europe, which promotes democracy and human rights across the continent.

The U.S. Embassy in Georgia also said it was disappointed by the vote result.

“We urge all Georgian stakeholders, including the government, all political parties, and civil society, to work cooperatively in a calm and respectful manner to move forward in line with our shared commitment to strengthening Georgia’s democracy,” the embassy said in a statement.

After Thursday’s protest, a number of opposition activists stayed outside parliament overnight and blocked traffic on the capital’s main Rustaveli avenue by erecting tents in the street.

Protests were held on Friday in two other cities and organizers called for a big rally in Tbilisi on Sunday.

“We should demonstrate our power on Sunday,” Nika Melia, an opposition United National Movement lawmaker, said.

The election is due in October next year but opposition leaders have joined forces to demand an earlier vote and the creation of an interim government in the meantime.

They said they suspected the failure of the electoral reform was orchestrated by Georgian Dream party head Bidzina Ivanishvili, a wealthy oligarch whom critics say governs the former Soviet republic of 3.7 million people behind the scenes.

Ivanishvili denied manipulating the process, saying he was “frustrated by the outcome”.