Two prominent Turkish journalists charged with aiding a shadowy group aiming to topple the Islamist-rooted government went on trial Tuesday as press freedom campaigners demonstrated outside.

Award-winning reporter Nedim Sener and dissident journalist Ahmet Sik were in the Istanbul court for their first hearing along with 12 other defendants in the case which has sparked international concern.

Turkish Prime Minister Recep Tayyip Erdogan on Tuesday called on his former ally Syrian leader Bashar Assad to step down.

"Quit power before more blood is shed ... for the peace of your people, your region and your country," Erdogan said in parliament.

Turkish police arrested more than 70 people in simultaneous raids around the country on Tuesday for suspected links to outlawed Kurdish rebels, media reports said.

Lawyers for jailed Kurdish leader Abdullah Ocalan who are members of the pro-Kurdish Peace and Democracy Party (BDP) were among those arrested, security sources from the southeastern province of Diyarbakir said.

Syrian leader Bashar al-Assad has reached a "dead end" but only internal forces can bring about change, Turkish President Abdullah Gul said Monday, in an interview with the Guardian.

Speaking on the eve of a three-day state visit to Britain, Gul warned that Assad's back-tracking on reforms agreed by the Arab League had made international negotiation impossible.

Turkish Prime Minister Recep Tayyip Erdogan warned Syrian President Bashar Assad Monday that his days as leader were numbered and he cannot remain in power indefinitely through military force.

After reports that two buses carrying Turkish pilgrims came under attack in Syria, Erdogan said his one-time ally's defiant refusal to end a bloody crackdown on protesters had increased the prospects of foreign intervention.

Two buses carrying Turkish pilgrims came under fire in neighboring Syria early Monday as they were travelling back from the hajj in Saudi Arabia, leaving two people injured, reports said.

The "armed attack" occurred between the flashpoint cities of Hama and Homs, Turkey's semi-official Anatolia news agency reported.

Turkish security forces foiled an attempted bombing blamed on Kurdish rebels on a vital oil pipeline in the southeast of the country on Sunday, local security sources said.

Three explosive charges weighing a total of 200 kilograms were found along the pipeline carrying oil from Iraq and were later defused by bomb disposal experts.

Turkish President Abdullah Gul headed to Britain on Sunday for a three-day state visit aimed at seeking support from the Turkey's ally in its bid to join the EU.

"I will underline the importance of England's continued support in making sure negotiations are not blocked by artificial political obstacles," Gul told journalists in Turkey before flying off to London.

Turkish President Abdullah Gul said there was "no place for authoritarian regimes" in the Mediterranean region, heaping more pressure on the embattled Syrian regime, in comments published Sunday.

"I strongly believe that there is no place any more for authoritarian regimes -- single party systems that do not have accountability or transparency -- on the shores of the Mediterranean," he told Britain's Sunday Telegraph newspaper.

Turkey warned Friday that the conflict in Syria risks turning into a civil war, while France's chief diplomat said it was now too late for the Syrian regime to finally implement reforms.

Turkish Foreign Minister Ahmet Davutoglu pointed to the new attacks by army defectors, "therefore I say there is a risk of transforming into civil war. It is now the right time to stop this massacre, and therefore the Arab initiative is important," he said.