Access to the Internet's most-used sites and tools is being choked in Iran at a politically charged period, blocking communication channels for local businesses, bank clients, scientists and foreign media.
Attempts to get on Facebook, Gmail, Yahoo and foreign news pages were either met with an Iranian page saying in Farsi that "Access to this page is a violation of computer crime laws" or the connection was slowed to such an extent to make it nearly impossible.
The restrictions add to the online censorship that authorities have long imposed in the Islamic republic.
Until now, sophisticated Internet users had been able to get around the blocks by using software known as a Virtual Private Network (VPN) -- the sale of which is illegal in Iran.
Since last week, though, even most VPNs offered no solution. Internet service providers (ISPs), under the control of the state, seemed to be targeting the Internet's most popular social networks and communication sites.
While authorities have given no reason publicly for the systematic curbs, the extra filtering came at a politically sensitive time for them.
Iran last week celebrated the anniversary of its 1979 Islamic revolution, and in less than three weeks' time it is to hold legislative elections.
A call has gone out -- on the Internet -- for "Green Movement" opposition demonstrations on Tuesday, exactly a year after protests that resulted in a severe crackdown and arrests.
On top of that, the Iranian government has said for some months that it is preparing to launch a "national Internet" that reportedly would exclude almost all non-Iranian or non-Muslim websites.
However, one member of the Iranian government's net filtering committee, Mohammad Sadegh Afrasiabi, denied the initiative was linked to the Internet problems.
"This network will only be put in place in four years' time and the blocking of emails isn't part of it," he was quoted by the Hamshahri newspaper saying.
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More broadly, Iran is in a worsening showdown with the West over its suspect nuclear programme. It has accused the United States and Israel of conducting covert operations against it, including through Internet subversion and the deployment of computer viruses.
Whatever the motives, the strangulation of the Internet is making life very difficult for many Iranian businesses, especially those needing to access emails and documents online through services such as Gmail.
Importers were "angry or desperate" at being cut off from communicating with suppliers abroad, said one source in the trade sector.
Businesses in Tehran were being forced to turn to fax machines and motorbike couriers to send or receive invoices and other documents, the source said.
"I'm waiting for urgent documents to prepare a contract with a Turkish company and my Internet is completely blocked," said the owner of a small Iranian electronic import firm who demanded anonymity.
The Hamshahri daily said the additional restrictions were "provoking disturbances in commercial and scientific exchanges in the country, with evident consequences even for urban traffic and for banking."
Foreign journalists and diplomats in Iran were also finding it difficult or impossible to communicate via the web, several told AFP.
The few Tehran cafes offering wi-fi connections regretfully told laptop- and smartphone-toting customers that their Internet service was off-line.
Iranians with family living abroad were despairing. "I haven't had any connection with the outside world for several days. I don't have any more contact with my son who works in France," said one mother living in Tehran.
A top conservative lawmaker in Iran, Ahmad Tavakoli, warned that "such annoying filtering will cost the regime dearly," according to the Mehr news agency.
"If there are justifications on security grounds, officials should explain them clearly to the people," he said.
But the head of one ISP, Mohammad Hassan Shaneh-Saz of the company Shatel, was quoted by Mehr saying the added restrictions were "not related to the quality of service from the ISPs."
He claimed that the proposed "national Internet" would improve the situation when launched, by increasing available bandwidth.
The Internet is widely used Iran, where nearly half the 75-million strong population is connected.
It played a major role in a wave of anti-government protests that rocked the country after the disputed re-election of President Mahmoud Ahmadinejad in 2009.