Turkey's 10 million Twitter users have been unable to access the site from late last night, following the PMs vow to 'eradicate it'.
The victim of a stabbing attack in Turkey has branded his attackers "disgusting", saying he owes his life to the "best surgeon in Turkey".
Anti-government protesters and police have been fighting a running battle for control of Istanbul's Taksim Square on an 11th day of clashes.
Twitter says it has filed petitions in Turkish courts to challenge the ban on its service.
In a statement the social network said:
"It’s now been six days since the Turkish government blocked access to Twitter. Throughout this time, we’ve been engaged in discussion with Turkish authorities ... But still, the millions of people in Turkey who turn to Twitter to make their voices heard are being kept from doing just that.
"So today, we filed petitions for lawsuits."
The Turkish Prime Minister has congratulated his armed forces after they shot down a Syrian plane, saying: "If you violate our border, our slap will be hard."
Recep Tayyip Erdogan said the plane violated Turkish air space and added: "I congratulate firstly our Chief of General Staff, and our pilots. I congratulate our Air Forces."
Syrian state television has quoted a military official saying the plane shot down by Turkey was pursuing rebel fighters within Syria's own territory.
The Turkish Prime Minister has said the jet was shot down because it had violated Turkish air space.
Turkey's Prime Minister, Recep Tayyip Erdogan, has confirmed that his country's armed forces shot down a Syrian jet after it violated Turkish air space.
A Syrian plane has reportedly been shot down while flying near the border with Turkey, an area where rebels have been fighting the forces of President Assad.
Turkey's NTV channel has quoted government officials saying the jet was shot down because it had violated Turkey's air space.
Twitter must appoint a legal representative in Turkey and block a number of individual accounts if the ongoing ban on the social media site is to end, the country's industry minister Fikri Isik has said.
Despite the block imposed yesterday, protesters have found ways to rail against Prime Minister Recep Tayyip Erdogan via the service.
Mr Erdogan vowed to "wipe out" Twitter in Turkey after alleged evidence of government corruption began to spread online.
Germany has joined a host of voices criticising Turkey's ban on Twitter.
"It doesn't fit with our idea of freedom of expression to forbid or block any form of communication," Chancellor Angela Merkel's spokeswoman said.
Turkey's courts blocked access to Twitter following Prime Minister Tayyip Erdogan's vow, on the campaign trail ahead of March 30 local elections, to "wipe out" the service.
He says he does not care what the international community says about it, though President Abdullah Gul has objected to his actions.
More than 500,000 tweets have been sent using the hashtag #twitterisblockedinTurkey since the service was banned in the country last night, according to social media search engine Topsy.
Analysis also shows 100,000 have used the hashtag #turkeyblockedTwitter at the time of writing.
European Commission vice-president Neelie Kroes has joined the criticism of Turkey's Twitter ban.
Turkey's president has hit back on Twitter against his prime minister's ban on the service.
Sosyal medya platformlar?n?n tamamen kapat?lmas? tasvip edilemez.
In a series of tweets, Abdullah Gul said that complete closure of social media sites was "unacceptable".
He added that only individual web pages should be shut down, rather than the service as a whole, should a court rule that privacy has been breached.
"Hopefully this application will not last long," President Gul said.
Ki?ilerin özel hayat?n?n gizlili?ini ihlal gibi suç olu?turan hususlar varsa, ancak mahkeme karar?yla sadece ilgili sayfalar kapat?labilir.