However Ahok immediately announced he would appeal the guilty verdict and sentence.
A Christian governor in Indonesia was sentenced to two years in prison for blasphemy against Islam, although he denies the charge.
"It's a sad day, and it's frightening", Andreas Harsono, an Indonesia researcher for Human Rights Watch, told the Times.
He added: "If someone like Ahok, the governor of the capital, backed by the country's largest political party, ally of the president, can be jailed on groundless accusations, what will others do?"
Indonesian Muslim protesters hold a rally during a trial for Jakarta governor Basuki Tjahaja Purnama outside the North Jakarta court on May 9, 2017.
Shocked and angry supporters, some weeping openly, gathered outside the prison, vowing not to leave the area until he was released, while others vented their shock on social media.
"Ahok will be jailed because he insulted religion", read one of many anti-Purnama posts on the Facebook account of the Islamic Defenders Front (FPI), a conservative group instrumental in organising mass protests against Purnama.
"They sentenced him because they were pressured by the masses. But, in the long-term investors will most likely take note of how the Jakarta court came to its decision under massive public pressure", Reza said.
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President Muhammadu Buhari will travel to London on Sunday night (tonight) for "medical follow-up,"his spokesperson has said". A military source said three Boko Haram commanders had been freed in an exchange, but declined to give further details.
Thousands of police were deployed in the capital early on Tuesday in case clashes broke out between Purnama's supporters and opponents.
Hardline Islamist groups initiated the process, accusing Ahok of insulting the Quran during his losing reelection bid a year ago and called for him to be arrested and tried.
The ruling significantly challenges the country's reputation as moderate in the Islamic world, particularly because prosecutors recommended a punishment of two years' probation, not imprisonment.
Hardline Islamist groups had called for the maximum possible penalty over comments by Ahok they deemed were insulting to the Islamic holy book, the Quran.
Ahok has been on trial for blasphemy since December following a speech he made in September in which he criticised his detractors for using a Koranic verse - Al Maidah 51 - to "fool" people into not voting for a non-Muslim. An incorrectly subtitled video of his comments later went viral, helping spark huge demonstrations that ultimately resulted in him being brought to trial.
Purnama lost his bid for re-election as governor in an April run-off - after the most divisive and religiously charged campaign in recent years - to a Muslim rival, Anies Baswedan. Purnama's deputy is expected to take over until Baswedan takes office in October.
The prosecutor last month called for the blasphemy counts to be dropped in exchange for a lesser charge of "spreading hate", but the judges Tuesday appeared to have ignored that recommendation.
The tensions whipped up during the Jakarta election have raised concerns about the rising influence of extreme groups in Indonesia, which is home to sizeable communities of Buddhists, Hindus, Christians and people who adhere to traditional beliefs.