ISTANBULISTANBUL (Reuters) - Turkey's main opposition leader launched a European court appeal on Tuesday over an April vote that granted President Tayyip Erdogan sweeping powers, stepping up his challenge to the government as he led a 425 km (265 mile) protest march.

Erdogan accuses the protesters, marching from Ankara to Istanbul, of "acting together with terrorist groups", referring to Kurdish militants and followers of a U.S.-based cleric who Ankara says was behind last year's coup.

Kemal Kilicdaroglu, head of the main opposition Republican People's Party (CHP), hit back on Tuesday, defending his "justice march" and accusing the government of creating a one-party state in the wake of the failed putsch on July 15.

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On the 20th day of his march, triggered by the jailing of a CHP deputy on spying charges, Kilicdaroglu signed an appeal to the European Court of Human Rights against the election board's decision to accept unstamped ballots in the April 16 referendum.

"Turkey has rapidly turned into a (one-)party state. Pretty much all state institutions have become branches of a political party," he told reporters. "This is causing profound harm to our democratic, parliamentary system."

Kilicdaroglu, 68, wearing a white shirt and a baseball cap with the word 'justice' printed on it, then set out on the latest leg of the march from the city of Izmit, around 100 km (60 miles) along the coast to the east of central Istanbul.

The protest has gained momentum as it passes through northwest Turkey's countryside and representatives of the pro-Kurdish HDP, parliament's third largest party, joined the march on Monday near the jail of its former co-leader Figen Yuksekdag.

There are deep divisions among opposition parties but Yuksekdag, stripped of her parliamentary status in February, issued a statement from her cell on Monday calling for them to put those differences aside.

"We must set up the shattered scales of justice again and fight for this together," she wrote, saying justice had hit "rock bottom" with the jailing of 11 HDP lawmakers and around 100 mayors.

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The party rejects charges of ties to the Kurdistan Workers Party (PKK) militant group, designated a terrorist group by Ankara and its Western allies, which launched an insurgency in 1984 in which more than 40,000 people have been killed.

"TERROR GROUP" ACCUSATIONS

As the protesters advance, Erdogan has stepped up his attacks on the march, saying the CHP was longer acting as a political opposition.

Turkey's main opposition Republican People's Party (CHP) leader Kemal Kilicdaroglu walks during the 19th day of a protest, dubbed ''justice march'', against the detention of the party's lawmaker Enis Berberoglu, near Kocaeli, Turkey, July 3, 2017. Reuters/Umit Bektas Supporters of Turkey's main opposition Republican People's Party (CHP) leader Kemal Kilicdaroglu walk with a giant Turkish flag on the 19th day of a protest, dubbed ''justice march'', against the detention of the party's lawmaker Enis Berberoglu, near Izmit, Turkey, July 3, 2017. Reuters/Umit Bektas Supporters of Turkey's main opposition Republican People's Party (CHP) leader Kemal Kilicdaroglu wave Turkish flags on the 19th day of a protest, dubbed ''justice march'', against the detention of the party's lawmaker Enis Berberoglu, near Izmit, Turkey, July 3, 2017. Reuters/Umit Bektas

"We can see that they have reached the point of acting together with terror groups and those powers which provoke them against our country," he said in a speech to officials from his ruling AK Party on Saturday.

"The path which you are taking is the one of Qandil, the one of Pennsylvania," he said, referring to the northern Iraqi mountains where the PKK is based and the U.S. state where Erdogan's ally-turned-foe Fethullah Gulen lives.

Kilicdaroglu launched his march in Ankara on June 15 after Enis Berberoglu was jailed for 25 years for espionage, becoming the first lawmaker from the party imprisoned in a government crackdown in the wake of the attempted coup.

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Since the purge began, more than 50,000 people have been jailed pending trial, 150,000 have been suspended or dismissed from their jobs. Ankara has also shut down 130 media outlets and some 160 journalists are in prison, according to union data.

In April a referendum was held on constitutional changes that sharply widened Erdogan's presidential authority and the proposals won 51.4 percent approval in a vote, which has triggered opposition challenges including the latest CHP move.

Opposition parties have said the poll was deeply flawed and European election observers said the decision to allow unstamped ballot papers to be counted had removed a main safeguard against voting fraud.