'Glitch' unblocks social media sites in Iran
17 September 2013 Last updated at 15:55 BST
Internet users in Iran were surprised to find themselves able to access Twitter and Facebook on Monday.
The websites have been blocked since 2009 when opposition protests followed the presidential elections.
Iran's government says a technical glitch led to the websites being unblocked. It appears that the firewall has returned and users are now unable to access the sites.
BBC World finds out how Iranians reacted, while BBC Persian's Siavash Ardalan explains what happened.