(Adds Zuckerberg statement)
By Natalia Scalzaretto and Caio Saad
SAO PAULO/RIO DE JANEIRO, May 3 (Reuters) - Facebook Inc's Chief Executive Mark Zuckerberg called on Brazilians to demand his company's WhatsApp messaging service never be blocked again after an appeals court on Tuesday overturned the application's second suspension in five months.
In a post in English on his Facebook page, the U.S. billionaire and Facebook founder urged Brazilians to gather outside Congress in the capital Brasilia at 6 p.m. (2100 GMT) on Wednesday to rally in favor of legislation that would prevent Internet services from being blocked.
WhatsApp was cut off in Brazil at 2 p.m. (1700 GMT) on Monday after a judge in the remote northeastern state of Sergipe ordered Brazil's five main wireless operators to block access to the app for 72 hours. The reason for the order was not made public.
The suspension of WhatsApp's text message and Internet voice telephone service for smartphones was lifted after about 24 hours when an appeals judge on Tuesday ruled in favor of an injunction by the company's lawyers, the court said in a statement. Some 100 million users were affected.
"You and your friends can help make sure this never happens again, and I hope you get involved," Zuckerberg wrote on Facebook. He also posted a link to a petition, calling efforts to block communication "very scary in a democracy."
The suspension highlighted growing international tensions between technology companies' privacy concerns and national authorities' efforts to use social media to gain information on possible criminal activities.
The same judge in Sergipe ordered the imprisonment of a Brazil-based Facebook executive in March in a dispute over demands to access the company's encrypted messaging service as part of a drug trafficking investigation.
California-based WhatsApp had said in a statement on Monday that it was "disappointed" at the judge's decision to suspend its services. It said it had done the utmost to cooperate with Brazilian tribunals, but it did not possess the information the court was requesting.
Matt Steinfeld, a Facebook spokesman, said WhatsApp executives were meeting this week with law enforcement and judicial officials in Brazil to improve communication and clarify that the company cannot see users' encrypted messages and does not store them after transmission.
It was the second time in five months that WhatsApp in Brazil has been suspended. A São Paulo state judge ordered it shut down for 48 hours on Dec. 15, after Facebook failed to comply with an order. Another court lifted that suspension shortly afterward.
Monday's suspension angered many in Brazil, where the service is used by individuals, companies and federal and local governments to send messages and share pictures and videos. Cost-conscious Brazilians are avid users of free messaging apps, and WhatsApp is by far the most popular - installed on more than 90 percent of Android devices.
As some Brazilians sought an alternative messaging system, rival Telegram said on Monday that it suffered technical problems under the weight of demand. It said it received more than a million new user requests.
Letícia Mendes, a 20-year-old shop assistant in Rio de Janeiro, said she was frustrated by the suspension because it could force people to use pay services.
"It's really bad," she told Reuters. "It's just a way of getting more money out of us, when we already have to pay for so many things."
The suspension came as a congressional commission on cyber crime in Brazil debated changes to the 2014 legislation governing the use of the Internet.
Lower house deputy Esperidião Amin, the rapporteur of the commission, said his proposed reform would help avoid shutdowns of this kind by allowing the blocking of specific individuals or IP addresses suspected of illicit activity, rather than the access of all users.
"It's less dramatic than withdrawing the service from the whole of the Brazilian population," he told Reuters by telephone. (Editing by Daniel Flynn and Cynthia Osterman)