The US is "deeply troubled" by a Turkish court's conviction of Istanbul Mayor Ekrem Imamoglu, who was given a jail sentence and banned from politics for insulting public officials.
The court sentenced Mr Imamoglu, a key member of the main opposition Republican People's Party, to more than two years in prison for calling members of Turkey’s Supreme Election Council “fools” three years ago.
His 2019 victory presented a strong challenge to President Recep Tayyip Erdogan’s Justice and Development Party, or AKP, and its grip on the city it controlled for more than two decades.
Mr Imamoglu is also considered to be the main presidential threat to Mr Erdogan in next June's elections.
The State Department said his conviction was "inconsistent with respect for human rights, fundamental freedoms, and the rule of law".
Washington also urged Turkey to stop prosecuting under its criminal insult laws.
"The people of Turkey deserve the ability to exercise their human rights and fundamental freedoms without fear of retribution," State Department spokesman Ned Price said.
Thousands, including leaders of opposition parties, gathered in protest outside the Istanbul municipal building on Thursday against the conviction.
At his headquarters across the Bosphorus Strait on the European side of Istanbul, Mr Imamoglu told his supporters that the verdict marked a "profound unlawfulness" that "proved that there is no justice in today's Turkey".
Voters would respond at the presidential and parliamentary elections, he said.
Mr Erdogan’s AKP has ruled Turkey since 2002.
After it initially passed some progressive reforms, the government showed "growing contempt for political rights and civil liberties", according to democracy group Freedom House.
The AKP has launched wide-ranging crackdowns on perceived opponents since an attempted coup in 2016.
In December, the European Court of Human Rights called for the release of Selahattin Demirtaş, leader of the Kurdish-aligned People’s Democratic Party, who was imprisoned in 2016 on what were widely believed to be politically motivated charges.
Ankara ignored the court’s ruling.
