ISTANBUL/ANKARA (Reuters) -- Aside from a carefully worded statement urging unity, President Tayyip Erdogan was unusually quiet after Turkey's worst ever bomb attack.

Modern Turkey's most divisive leader has in the past had no hesitation in dominating the airwaves at times of crisis, rallying his fervent supporters and lambasting his opponents in equal measure in defense of the state.

But the double suicide bombing that killed up to 128 people at a rally by pro-Kurdish and leftist activists Saturday, three weeks before an election, has sparked criticism of Erdogan's administration just as Turkey is already beset by conflict in its Kurdish southeast and seemingly paralyzed by the growing spillover from Syria's war.

For those loyal to Erdogan and the Islamist-rooted AK Party he founded, the bombings marked another murky conspiracy by foreign-backed forces to undermine the Turkish state and damage its standing in the Middle East.

For his opponents, including the pro-Kurdish opposition party apparently targeted in the blasts, his administration has blood on its hands -- at best for intelligence failings, at worst for complicity in a bid to stir up nationalist sentiment.

"Once the initial shock has subsided, the attack appears likely to exacerbate the already deep cleavages in a dangerously divided society," Wolfango Piccoli, managing director of London-based Teneo Intelligence, said.

Investigations are focusing on the Islamic State group, senior security sources told Reuters, although Prime Minister Ahmet Davutoglu said in the hours after the blasts that any of a plethora of radical groups could have been responsible, from Islamist or Kurdish factions to far-leftists.

The Islamic State has openly claimed past attacks, sometimes opportunistically taking responsibility for actions it did not direct. There has been no such claim for the Ankara bombing, and skeptics see the group as a convenient scapegoat.

Sidelining Turkey?

Those close to Erdogan see the bombing as a calculated bid to further weaken him at a time when his strategy in Syria of seeking President Bashar Assad's overthrow and curbing Kurdish territorial gains has been left in disarray by Russia's intervention in support of Assad.

"This attack was carried out by those who want to leave Turkey and Erdogan, along with the AKP, out of the equation in the Middle East. Intelligence organizations influential in the region must have given support," one senior government official told Reuters, without elaborating.

Erdogan's opponents have similarly elaborate conspiracy theories.

"Many in Turkey will suspect that pro-government clandestine forces may be somewhat complicit ... as part of a 'strategy of tension' aimed at scaring voters into supporting Erdogan’s law-and-order, security-first platform," Piccoli said.

In a statement via his office Saturday, Erdogan condemned the bombing and called for "solidarity and determination as the most meaningful response to terror." Officials rejected any suggestion he was not in control.

"It's meaningless to hold the AKP, Erdogan or the government responsible for what has happened," one senior AKP official said. "Erdogan and the government are aware of their responsibility. They are focused on doing what is necessary. The rest is not important."

Psychological Impact

The psychological impact of such a devastating attack in the heart of the capital is clear.

"How can such a big bomb explode in the middle of this country's capital city?" Selin, a 32-year-old teacher attending an anti-government protest in Ankara Sunday, said.

"The intelligence agency, the police and security officials must be too busy finding and jailing people who send critical tweets against Erdogan," she said. The editor-in-chief of the English-language newspaper Today's Zaman was detained Friday on charges of insulting Erdogan on Twitter.

Even some AKP sympathizers may question their loyalty after the Ankara attacks, said Sinan Ulgen, a visiting scholar at Carnegie Europe and head of the Istanbul-based EDAM think tank.

"The government is ultimately responsible for the protection of its citizens, and that responsibility cannot be pushed away. There is a security and intelligence failure here. That you can't hide," he told Reuters.

But he said the impact on the voting intentions of an electorate where divisions are deeply entrenched was likely to be minimal.

That means the AK Party will, in the view of most analysts, fail to claw back the majority it lost in an election in June after more than a decade of single-party rule. That result was a direct blow to Erdogan's hopes for a more powerful executive presidency, which rest on the party he founded once again dominating Parliament.

Election Security Worries

Guaranteeing the security of the Nov. 1 election will inevitably be a more pressing issue now.

Concerns had focused on the predominantly Kurdish southeast, where conflict between the security forces and Kurdistan Workers' Party militants has flared anew in recent months.

A bomb blast at a campaign rally of the pro-Kurdish Peoples' Democratic Party (HDP) killed four people in the town of Diyarbakir on the eve of the last election, while a suicide bombing in July in the town of Suruc near the Syrian border killed 34 people, mostly young pro-Kurdish activists.

Saturday's attack showed that the threat is by no means limited to the southeast, but government officials made clear that postponing the election was out of the question.

The HDP also wants the vote to go ahead, eager to boost its representation after entering parliament for the first time in June, by expanding beyond its Kurdish voter base and drawing in mainly left-wing opponents of Erdogan.

"We're eager for the election, whereas the dictator in the palace is fleeing the election," HDP honorary president Ertugrul Kurkcu said, reflecting a widely held suspicion among Erdogan's opponents that he would like to postpone the election until he can be sure of stronger AKP support.

"We could consider various changes in our campaign style to prioritize security. We haven’t clarified that yet, but we're aware that a new situation has emerged after the attack."

While the election looks likely to return a hung parliament again, some of Erdogan's supporters think prolonged instability could have voters yearning for a return to strong single-party rule further down the line.

"This country needs peace and stability. No one has the right to write this country's history in blood," said Tayfun Kilic, a 40-year-old civil servant, waiting for a bus in a central Ankara square.

"I hope we will have a single-party government after the election, because our country needs unity and a stronger, more decisive fight against terrorism."