Hours after chat app WhatsApp was blocked in Brazil, on orders of a Sao Paulo judge, Facebook co-founder and CEO (Facebook owns WhatsApp) Mark Zuckerberg posted a public note, calling the blockade a "sad day for Brazil."

"Until today, Brazil has been an ally in creating an open internet. Brazilians have always been among the most passionate in sharing their voice online," he wrote on his Facebook profile.

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The block, which started on Thursday, at midnight local time, is scheduled to be in effect for 48 hours. According to Reuters, it comes after a party sought an injunction before a criminal court in Sao Paulo, but that party's name was not released to the public.

It's worth noting that Brazilian telecoms tried to curb the use of WhatsApp's VoIP services in the country in August, claiming it hurts their profits.

This is not the first time a Brazilian judge has ordered a Facebook blockade in the country. In 2013, a Sao Paulo judge said the site would be shut down if a "defamatory" post was not removed; Facebook complied within 48 hours, avoiding the shutdown.

In his note, Zuckerberg did not say the exact reason for the blockade, but hinted that it has something to do with protection of users' privacy.

"I am stunned that our efforts to protect people's data would result in such an extreme decision by a single judge to punish every person in Brazil who uses WhatsApp," he wrote.

According to Zuckerberg, the company is "working hard" to get the block reversed. He also pointed out users in Brazil can still communicate via Facebook Messenger.