Previous elections have seen protests organised via social media sites such as Twitter and Facebook
Iranians are struggling with slower Internet speeds and limited access ahead of an unpredictable presidential election that has put hardline Islamist authorities on alert for possible unrest.
Experts and web users say they believe the Internet obstacles are related to the June 14 presidential vote, the first since 2009 polls in which accusations of fraud - denied by the government - kindled months of protests organised in part via social networking sites like Facebook and Twitter.
Iranian officials denied any connection between the Internet disruptions and the upcoming vote. But, after the disputed re-election of President Mahmoud Ahmadinejad four years ago, they are wary of the possibility of further unrest this time around.
The last-minute entries of moderate former president Akbar Hashemi Rafsanjani and Ahmadinejad ally Esfandiar Rahim Mashaie have shaken up what was expected to be a limited race between hardline conservatives close to clerical Supreme Leader Ayatollah Ali Khamenei and hostile to Ahmadinejad.
The populist Ahmadinejad, who has fallen out with Khamenei, is limited by law to two consecutive presidential terms.
Iran's Guardian Council was due to present a final list of approved candidates to the Interior Ministry on Tuesday. The ministry then has two days to announce the approved names.
The opposition website Kaleme reported on Monday that security had been heightened in Tehran, apparently to counter any protests should the candidacies of Rafsanjani or Mashaie be rejected by the council.
Iranian web users, who number some 45 million according to official figures, have grappled with increased obstacles to using the Internet since the 2009 election.
Kaleme said on Monday Internet speeds had dropped in much of Tehran and that in some parts of the capital, accessing the Web had become impossible - which would prevent dissidents from mustering protests online as they did after the 2009 vote.
Hamed, 33, a dissident freelance journalist living in Tehran, said his clients now have resorted to sending him files by loading them onto CDs and transporting them by courier.
"We get things done but with more time spent," Hamed told Reuters via email.
Many Iranians used Virtual Private Network (VPN) software to bypass the government's extensive web filter. But the government blocked access to most VPNs, which make computers look as if they are located in another country, in March.
Since then, experts said, Iranians have faced slower access to encrypted international websites using the Secure Sockets Layer protocol, with addresses beginning with "https", such as Google Inc.'s email service Gmail, and this could push them to resort to unencrypted sites easily watched by the state.
"SSL services are being throttled by the government to create a system of incentives or coercion not to use them," said Collin Anderson, a US-based Internet researcher who focuses on Iran. "That affects Gmail and pretty much anything that you want a layer of security on."
A similar Internet blockade was put in place in February 2012, ahead of parliamentary elections.
Several Iranian Web users said they have had trouble accessing their Gmail accounts in the last three weeks. Elham, an Internet user from the northeastern city of Mashhad, told Reuters that since late April, any VPN she tries to use only works for about two minutes before she is disconnected.
She and Hamed declined to be fully named for fear of repercussion for speaking to a foreign reporter.
One man in his 30s who works at an Internet Service Provider (ISP) company in Tehran confirmed that most VPNs were down and those still up were crashing within two minutes.
He added: "Fewer and fewer people are using Twitter in recent days which shows their problems accessing the net."
Iranian officials denied any link between the disruptions and the election. "Numerous parameters contribute to the speed of the Internet and the approach of elections will not have any role," Ali Hakim Javadi, head of Iran's Information Technology Organisation, told ISNA news agency.