Turkey's main opposition party has launched a legal bid in an EU court over the referendum that expanded President Erdogan's powers. CHP head Kemal Kilicdaroglu is also staging a "justice march" from Ankara to Istanbul.
"The YSK decision made the referendum illegitimate. We are applying to the European Court of Human Rights," Kilicdaroglu told reporters in the city of Izmit, around 100 kilometers (62 miles) east of central Istanbul.
Kilicdaroglu is expected to complete the 425-kilometer (265-mile) trek this coming weekend.
DW's Turkey correspondent, Dorian Jones, said the protest had gained momentum as it approached Istanbul.
"The fact that this march is strictly non-political - there are no political banners - it appeals to all sections of society," he said.
"It's inclusive and this is something that's rarely seen in the polarized world of Turkish politics. This is crossing the divide and it's certainly causing big concerns for the president and his government."
President Erdogan accuses the protesters of "acting together with terrorist groups," referring to Kurdish militants and supporters of US-based cleric Fethullah Gulen, who Ankara blames for masterminding the coup.
The march, and the European court bid, come at a time of heightened tensions as Turkey commemorates one year since the coup attempt this week.