By Evren Ballim and Paul Sandle
ISTANBUL/LONDON (Reuters) - Turks are turning to encryption software to thwart any ramp up in censorship of the Internet after six days of anti-government demonstrations and a wave of arrests reportedly for urging people to protest on social media sites.
Hotshot Shield, a VPN (virtual private network) that disguises users' identities and encrypts traffic on the Web, said more than 120,000 people had signed up to its service in Turkey since the weekend, more than 10 times typical levels.
The software has been used in recent years by democracy movements around the world, including in the Arab Spring, to circumvent government censorship of social media services such as Facebook and Twitter, said David Gorodyansky, founder of Hotshot Shield creator AnchorFree.
Authorities in Egypt, Libya and Syria attempted to close down Internet access completely to quell protests.
Gorodyansky said authorities had not blocked access in Turkey, but they had "throttled down" speeds, making the sites unusable for periods of time.
Police raided 38 addresses in the western port city of Izmir and detained 25 people on suspicion of stirring insurrection on social media with comments on the protest, opposition CHP party deputy Alaattin Yuksel told Reuters on Wednesday.
Izmir Deputy Prosecutor Ali Haydar confirmed that a detention order was issued for 38 people, but declined to give information on charges or how many were detained.
Before the arrests, protesters in Istanbul had voiced suspicions that the Internet was being restricted.
"We had problems accessing Twitter and Facebook Saturday when the protests were intense," said Deniz Utku, a digital marketing agency founder whose office is close to Taksim Square, the center of the demonstrations.
"I used VPN to access social media for 1.5-2 hours. I don't think this (slowness) was caused by the high demand ... We couldn't open the pages without VPN."
The government has made clear its disapproval of social media services, which are being used more and more as newspapers and television come increasingly under the sway of the state.
In a television interview last week, Prime Minister Tayyip Erdogan described sites such as Twitter as a "scourge", saying they were used to spread lies about the government with the aim of terrorizing society.
Regulators and operators, however, said no sites had been blocked. "No restrictions have been made in accessing social media sites," Tayfun Acarer, head of the telecommunication regulator Information and Communication Technologies Authority (ICTA), told Reuters.
Turkcell, the country's leading operator, said it had not received any request to block mobile communications in any part of Turkey. "Our technical teams are working to provide uninterrupted communications services as always," the company said on Twitter.
Industry experts said thousands of protesters in Istanbul would have put a heavy strain on networks.
Vodafone, a British company that was criticized when it complied with government demands to switch off service in Egypt along with other operators in 2011, said it had increased capacity in Turkey to meet demand for social media, in particular for sending images and video over mobiles.
Turks have also turned to other smartphones apps that can be used to maintain communications in case of a clampdown, such as Zello, which allows an iPhone to function like a walkie-talkie. It is currently a top-ranking app, according to App Annie analytics.
"The really interesting thing here is that tens of thousands of people are downloading Hotspot Shield and other communication apps in anticipation of further censorship," said AnchorFree's Gorodyansky.
"It just goes to show how evolving internet and mobile app technology is helping to thwart attempts to limit democratic rights and freedoms."
(Additional reporting by Ozge Ozbilgin from Ankara; Editing by Will Waterman)