The European Parliament has called for Turkey's European Union accession talks to be suspended if Ankara fully implements plans to expand President Tayyip Erdogan's powers.
Turkey on Thursday dismissed the vote as flawed and wrong.
The parliament has limited influence on Turkey's decades-old pursuit of EU membership, now in limbo after bitter exchanges between Ankara and some European countries, but the decision highlighted the gulf which has grown between the two sides.
EU leaders have been critical of Mr Erdogan and his behaviour toward opponents, both before and after an abortive military coup against him last July.
A year-long crackdown since the failed coup and the sweeping new powers which Erdogan won in a tightly fought referendum in April have raised concerns among Turkey's Western allies.
Mr Erdogan says both the crackdown and the increased presidential powers are needed to help tackle serious challenges to Turkey's security both at home and beyond its borders.
The resolution passed by parliament in Strasbourg "calls on the Commission and the member states... to formally suspend the accession negotiations with Turkey without delay if the constitutional reform package is implemented unchanged."
Some constitutional changes approved in April have already been implemented - Mr Erdogan has been able to return to lead the ruling AK Party, and members of a top judicial body have been changed.
Other steps, such as scrapping the post of Prime Minister, are due to take place within two years.
Opposition parties and human rights groups say the changes threaten judicial independence and push Turkey towards one-man rule.
Turkey's EU Affairs Minister Omer Celik said Ankara regarded Thursday's vote in Strasbourg as invalid, while the foreign ministry was similarly dismissive.
"This decision, which is based on false claims and allegations, is trampling the reputation of the institution in question," the ministry said in a statement, referring to the European Parliament. "This decision is of no value for us."
Despite their concerns over Turkey, EU leaders do not want to undermine an agreement struck last year whereby Ankara effectively stopped migrants reaching Greece from Turkish shores, thereby easing a crisis that had threatened EU unity.