With the country in political turmoil pitting the secular forces and the ruling Justice and Development Party (AKP) against each other, and the army fighting Kurdish rebels, alleged al-Qaida terrorism and accusations of coup attempts, the twin bombing could have been the doing of a number of groups ranging from Islamist militants, to extreme right-wing and leftist groups.

While the authorities have not been specific about the suspected parties behind the deadly explosions, security sources and politicians implicitly pointed the finger at the Kurdistan Workers Party (PKK), saying the type of bombs were the same as those used by the Kurdish rebels in the past.

The PKK had claimed responsibility for a May 2007 blast in Ankara, which killed six people and wounded some 100 others. The local media quoted police sources as saying the same type of devices were used in Sunday night's attack.

Prime Minister Recep Tayyip Erdogan, who flew from Ankara to Istanbul to visit the scene of the blasts after canceling a scheduled cabinet meeting, all but named the PKK, saying the bombings were "the cost" of the military crackdown against the group in southeast Turkey and northern Iraq.

"Unfortunately, the cost of this [military action] is heavy," Erdogan said at the scene in Turkey's largest city. "The incident last night was one such example."

Both bombs were planted in concrete garbage containers on a crowded pedestrian street in the popular Gungoren neighborhood in Istanbul's European side.

The first blast went off at 10 p.m. Sunday, and the second more powerful one exploded 10 minutes later, about 50 meters away after people had gathered around the first one. All the casualties, including five dead children, were victims of the second blast.

Turkish commentators argue that the target was a classic PKK choice for its campaign against the government, ruling out al-Qaida, which was accused of a series of attacks in Istanbul in 2003 that killed 63 people.

The attacks five years ago were all suicide bombings and targeted the British consulate, the British HSBC bank and two synagogues.

Istanbul Governor Muammer Guler said initial investigations showed there was a "link with the separatist organization," in reference to the PKK, which in 1984 launched an armed campaign for Kurdish autonomy and led to a conflict that claimed more than 37,000 lives.

But the movement, considered a terrorist group by Turkey and much of the rest of the world, has denied any involvement for Sunday night's bombings.

"The Kurdish liberation movement is not involved in this attack," the pro-PKK Firat news agency quoted Zubeyir Aydar, a senior group activist, as saying.

He accused "sinister forces" of carrying out the attack to coincide with the ruling party's trial and the pending case against the ultra-nationalist "Ergenekon," a shadowy group claimed to have organized attacks and plotted assassinations to create chaos and prompt a military coup against the ruling AKP.

The authorities have arrested an unknown number of people allegedly belonging to Ergenekon, including top former military officers, lawyers and politicians, on charges of terrorism plots and conspiracy to overthrow the government.

Meanwhile, Turkey's constitutional court, made up of 11 judges, convened on Monday to consider whether to ban the mildly Islamist AKP, led by Erdogan and includes President Abdullah Gul, on the grounds that it is allegedly injecting Islamic laws into the country's strictly secular system.

The prosecutor had asked the court to outlaw the party and ban Gul and Erdogan, as well as 69 AKP officials, from party politics for five years – something that politicians warn would destabilize the country and plunge it into a serious crisis.

The AKP, which has its roots in previously banned Islamist parties and won an overwhelming re-election victory with 47 percent of the vote in 2007, says it is facing a "judicial coup" to oust it from power by the hardcore secular forces that dominate the judiciary, army and academia.

If the judges ignore the court-appointed rapporteur's non-binding recommendation last week to acquit the party of the charges and rules to dissolve the party, Turkey's bid to join the European Union could also be adversely affected.

The constitutional court, which has banned 24 parties since its establishment 45 years ago, could endorse the prosecution's call, or cut treasury aid to the party, or throw out the case altogether.

Many observers believe the court's record shows it would likely disregard the possible repercussions that could come with banning the popular party, especially after the same judges had overturned a parliamentary bill amending the constitution to lift a ban on Muslim headscarves in universities.

Erdogan, speaking from the bombing scene on Monday, indicated that the continued rifts in the country would encourage further attacks, saying that "the strongest response our nation will give to this attack … will be to strengthen our unity."