Ai's lawyer was present for Friday's ruling, which had been widely expected, after the Beijing Chaoyang District Court barred the 54-year-old from attending.
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The 15m yuan fine was imposed on Ai's company, Beijing Fake Cultural Development, in November – and the artist, who was held in secret detention for 81 days amid calls for anti-government protests last year, maintains it is politically motivated.
In a Twitter post following the ruling, Ai said his company would “keep appealing, until the day comes when we have nothing to lose.”
Ai had challenged the tax fine on the grounds that proper procedure had not been followed, the BBC reported.
Speaking to journalists outside his Beijing studio, Ai said:
"China keeps telling other countries they are a rule of law country... But we only hope they implement the laws they themselves drew up."
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Media reports said the court was under tight police guard, with journalists and diplomats unable to enter as the verdict was read.
Ai criticized the decision to block him from the proceedings, and following the verdict he described the Chinese judiciary as being “shrouded in darkness."