ISTANBUL — Thousands of people gathered outside the Istanbul municipal building for a second day on Thursday to denounce a legal verdict that could lead to the city’s popular mayor being ousted from office and barred from running in elections next year.

An Istanbul court on Wednesday convicted Mayor Ekrem Imamoglu, 52, of insulting members of Turkey’s Supreme Electoral Council. The court sentenced him to two years and seven months in prison and imposed a political ban.

Opposition parties, which have questioned the independence of the courts under President Recep Tayyip Erdogan’s increasingly authoritarian rule, allege Imamoglu’s prosecution and trial were an attempt to eliminate a key opponent to the Turkish leader.

The mayor plans to appeal the verdict and is expected to remain in office while the case is under review by a higher court.

Erdogan, who has been in power since 2003, first as prime minister and then as president, is seeking a new five-year term in office in a presidential election currently scheduled for June. He has seen a dip in his approval ratings amid economic turmoil and soaring inflation.

An alliance of six opposition parties, including Imamoglu’s center-left Republican Peoples’ Party, has yet to nominate a candidate, but polls indicated the mayor had the potential to unseat Erdogan.

The leaders or representatives of the six parties were taking part in Thursday’s rally in support of Imamoglu, whose conviction sparked international reaction.

New York-based Human Rights Watch called the court’s decision a violation of the mayor’s rights as well as an “unjustified and politically calculated assault on Turkey’s political opposition” in the run-up to the 2023 election.

“The verdict against Ekrem İmamoglu is a travesty of justice and an attack on the democratic process, demonstrating that as the 2023 elections approach,the government is prepared to misuse courts to sideline or silence key opposition figures,” Tom Porteous, the advocacy group’s deputy program director said.

The U.S. State Department said Wednesday it was “deeply troubled and disappointed” by the trial’s outcome. “This unjust sentence is inconsistent with respect for human rights, with respect to fundamental freedoms and the rule of law,” State Department deputy spokesman Vedant Patel said.

Imamoglu maintained he was targeted because of his success as mayor in Turkey’s most populous city.

“Sometimes in our country, no success goes unpunished,” he said during a ceremony to open a care center for older adults. “I therefore regard this meaningless and illegal punishment given to me as a reward for my success.”

Imamoglu was elected mayor of Istanbul in March 2019, dealing a historic blow to Erdogan’s Justice and Development Party, which had controlled Istanbul for around a quarter-century.

The ruling party pressed for an annulment of the election results in the city of 16 million, citing alleged irregularities. The challenge resulted in a repeat of the election a few months later, which Imamoglu also won.

The mayor was charged with insulting senior public officials after he said in comments to journalists on Nov. 4, 2019 that canceling legitimate elections was “foolishness.”

Imamoglu denied insulting electoral council members, insisting that his words were a response to Interior Minister Suleyman Soylu, who had called him a “fool.”