Twitter has apologised for suspending accounts which have criticised the Chinese government on the eve of the 30th anniversary of the Tiananmen Square protests.
No precise casualty figures exist for the protests, which concluded with a brutal military crackdown on 4 June 1989.
Official figures, which are likely understated, report that around 300 people died while 7,000 were wounded.
Despite this, discussion of the protests is forbidden in China. The country's sophisticated internet censorship apparatus blocks any reference to the protests online, and they are not recorded in textbooks.
In a series of statements over the weekend, Twitter said that it suspended "a number of accounts" for violating rules about inauthentic behaviour, prompting concerns about it facilitating state censorship in China.
"Some of these were involved in commentary about China," the social media platform acknowledged, but added: "These accounts were not mass reported by the Chinese authorities – this was a routine action on our part."
Twitter added: "Sometimes our routine actions catch false positives or we make errors. We apologise."
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Users concerned by the bans suggested that the Chinese government may have had a hand in manipulating the target accounts to make them appear as if they were in breach of Twitter's rules.
The bans also drew sharp criticism from US Senator Marco Rubio.
Mr Rubio called on Twitter to answer "why it appears that Chinese dissident voices have been silenced just days before the 30th anniversary of the Tiananmen Square massacre".