Erdogan was speaking at a rally in Istanbul on Saturday to mark one year since a failed coup attempt that left 240 dead. 

"First of all we will chop off the heads of those traitors," Erdogan told the rally.

Read more: Questions abound, one year after Turkey's coup that wasn't

This prompted calls from the crowds that capital punishment should be restored, to which he said: "I will sign it" if parliament passes a bill on resuming executions, a move that would likely end any lingering hopes Ankara has of joining the EU.

"I spoke to the prime minister and [...] when they appear in court, let's make them appear in uniform suits like in Guantanamo," Erdogan added.

"They showed no mercy when they pointed their guns at my people," Erdogan said. "What did my people have? They had their flags - just as they do today - and something much more important: They had their faith."

"Nobody who betrays this nation can remain unpunished."

The Turkish president later flew back to Ankara to take part in late-night commemorations in the capital. Erdogan spoke outside of the parliament building, which had been bombed by warplanes during the coup attempt, telling thousands of supports in the crowd: "Our nation showed the whole world what a nation we are on July 15."

Read more: Turkey referendum: Could Erdogan resume executions?

Erdogan also repeated calls in Ankara that he would approve "without any hesitation" any legislation that would reinstate capital punishment in Turkey.

Cellphone users forced to hear message

Anyone who made a mobile phone call in Turkey on Saturday was forced to listen to a recording of Erdogan conveying them his good wishes first.

Omer Fatih Sayan, the head of the information and communications authority, confirmed Erdogan recorded the message.

Erdogan is heard saying in his message: "As your president I congratulate you on the July 15 Democracy and National Unity Day, I wish God's mercy and grace for our martyrs and good health for our veterans."

'Darkest night'

Lawmakers and Erdogan had earlier gathered in the parliament to remember the night of July 15 when thousands of unarmed civilians took to the streets to defend Erdogan, who has ruled Turkey since 2003, first as prime minister and since 2014 as president.

Prime Minister Binali Yildirim called the coup attempt a dark moment for Turkey, with deadly clashes between the people and rogue military forces.

"It has been exactly one year since Turkey's darkest and longest night was transformed into a bright day, since an enemy occupation turned into the people's legend," he said.

"Our people did not leave sovereignty to their enemies and took hold of democracy to the death," he went on, as Erdogan and members of opposition parties looked on. "These monsters will surely receive the heaviest punishment they can within the law."

Later in the day, tens of thousands of people, many waving red Turkish flags, took part in a march converging on the July 15 Martyrs' Bridge.

Erdogan earlier joined the crowds and unveiled a memorial to honor those who died opposing the coup.

Deep-lying tensions

But the cordial ceremonial unity belies tensions barely beneath the surface. Beyond the groundswell of nationalism, the coup's greatest legacy has been the far-reaching purge on multiple sectors of Turkish society.

About 150,000 people have been sacked or suspended from jobs in the civil service and private sector. More than 50,000 have been detained for alleged links to the putsch.

A fresh wave of firings came on Friday, when the government announced it had dismissed another 7,000 police, civil servants and academics for suspected links to the Muslim cleric it blames for the putsch.

Turkey's jailed journalists

Erdogan and his government have repeatedly blamed US-based cleric Fethullah Gulen for the coup plot, though they have offered little evidence to substantiate their claims.

Government critics, including human rights groups and some Western governments, have accused Erdogan of using the state of emergency introduced shortly after the coup to target opposition figures including activists, pro-Kurdish politicians and journalists.

The pro-Kurdish Peoples' Democratic Party (HDP) was represented by its deputy chairman at Saturday's events because the party's two co-leaders are in jail - as are local members of the human rights group Amnesty International.

The Committee to Protect Journalists says nearly 160 journalists have been arrested over the past year, making Turkey the largest jailer of journalists in the world.

During the ceremony in parliament, the head of the main opposition Republican People's Party (CHP) slammed what he called the erosion of democracy following the coup.

"This parliament, which withstood bombs, has been rendered obsolete and its authority removed," said Kemal Kilicdaroglu, referring to an April referendum that Erdogan narrowly won, giving him sweeping executive powers.

"In the past year, justice has been destroyed. Instead of rapid normalization, a permanent state of emergency has been implemented," he added.

Earlier this month Kilicdaroglu finished a 25-day, 425 km (265 mile) "justice march" from the capital Ankara to Istanbul, to protest the detention of a CHP lawmaker. The march was largely ignored by the pro-government media, but culminated in a huge rally in Istanbul against the government crackdown.

bik/tj/jbh/dm/aw (Reuters, dpa, AP)