Turkish President Recep Tayyip Erdogan thanked his allies on Sunday night for his self-proclaimed victory in the country’s historic referendum, even as opposition parties vowed to challenge the results. With 98 percent of votes counted Sunday night and 51.3 percent of votes in favor of constitutional changes, Erdogan’s supporters took to the streets to celebrate, setting off fireworks in Istanbul. While Erdogan said he was “grateful” to all those Turks who “reflected their will” by voting yes, his opponents have expressed concerns about how the referendum was conducted. In an unprecedented move during the vote, the election board agreed to accept ballot papers lacking the official stamp as long as there was no evidence the ballots were brought in from elsewhere. The country’s main opposition party, the Republic People’s Party, has already vowed to challenge a large number of the votes, saying the ballots issue poses a “legitimacy problem” for the referendum. The pro-Kurdish opposition has also said it would appeal two-thirds of the vote. The changes set to take effect after the referendum are the most drastic constitutional amendments since the country became a republic, placing almost all power in the hands of the president alone. Erdogan has praised the move as a way to ensure peace in a country plagued with security problems, while critics warn that the changes will lead to one-man rule and tyranny.