Six people have been killed and 81 injured after an explosion on Istanbul’s popular pedestrian thoroughfare İstiklal Avenue, in a bomb attack that Turkey’s president described as an act of terrorism. Speaking shortly before departing for Tuesday’s G20 summit in Bali, Recep Tayyip Erdoğan spoke of a “treacherous attack”, adding: “Those responsible will be punished.” Erdoğan said investigations were continuing but initial reports suggested an act of terrorism. “The first information provided to us by the [Istanbul] governor suggests this was an act of terrorism,” he said. His vice-president, Fuat Oktay, said a female attacker had detonated a bomb on the busy shopping street. “Whoever is behind these events, they will be found, even if they go to the other end of the world,” he added. The explosion occurred at about 4.20pm local time (13.20 GMT) on Sunday, the Istanbul governor, Ali Yerlikaya, tweeted. While central Istanbul has been targeted by Kurdish separatists and militant Islamists in the recent past, the attack has so far not been claimed. Five prosecutors had been assigned to investigate the explosion, the state-run Anadolu news agency said. The explosion shook buildings around the busy tourist area, rattling windows and sending flocks of pigeons into the air. Videos posted online from the moment of the attack showed terrified people running and trying to seek cover in nearby shops as a fireball billowed overhead in the distance from the middle of the street. Shoppers who had previously been strolling in the afternoon sun clutched each other in fear before turning to run away. “When I heard the explosion, I was petrified, people froze, looking at each other. Then people started running away. What else can you do?” said Mehmet Akus, 45, a worker in a restaurant on the avenue. “My relatives called me, they know I work on İstiklal. I reassured them,” he told Reuters. CCTV footage from the time of the attack distributed by Reuters shows tens of pedestrians at a crowded point in the busy thoroughfare, moving between concrete benches and the surrounding shops when the explosion struck. As the smoke clears, at least four motionless bodies can be seen on the ground while people who were knocked down attempt to get up and run away. Other footage showed a few people scattered around the site of the explosion, in front of a large Mango clothing store, with some rushing to help. Ambulances, fire engines and police were at the scene. An hour after the explosion, parts of the avenue remained open to pedestrians, while shops were shuttered and police helicopters flew overhead. At a police cordon to block off the area immediately around the explosion, families and tourists tried to hurry away as security services locked down access to the section of the street leading to the central Taksim Square. Turkey’s media ombudsman, RTÜK, placed a temporary ban on reporting of the explosion, preventing broadcasters from showing the moment the blast struck or the immediate aftermath, “to avoid broadcasts that may create fear, panic and turmoil in society and may serve the purposes of terrorist organisations”. Social media users reported that access to Twitter was throttled. The web freedom monitoring organisation NetBlocks said network data showed the Turkish authorities were restricting access to social media platforms including Twitter, Instagram, YouTube and Facebook after the attack. “Access to information is vital in times of emergency. Research shows that social media restrictions increase misinformation after security incidents and attacks,” said the NetBlocks founder, Alp Toker. The Turkish parliament recently passed a sweeping new law prohibiting “disinformation”, under which social media users or journalists accused of breaking it could be jailed for up to three years. The head of RTÜK, Ebubekir Şahin, warned citizens against spreading misinformation about the attack. “We are following the developments closely. Please do not rely on information from unclear sources. Let’s get information from reliable sources. Let’s not spread false information unknowingly,” he said. İstiklal Avenue is a crowded street popular with tourists and local people, lined by shops and restaurants. The Istanbul mayor, Ekrem İmamoğlu, of the opposition Republican People’s party (CHP), visited İstiklal Avenue following the explosion, which occurred less than 200 metres from the party’s Istanbul headquarters. “I received information from our fire brigade teams on İstiklal street. They continue their investigations in the area in coordination with the police. Our healthcare workers are also working hard. I wish God’s mercy once again to those who lost their lives, and I wish a speedy recovery to the injured,” he tweeted. Turkey was hit by a string of deadly bombings between 2015 and 2017 by Islamic State and outlawed Kurdish groups. They included a suicide bomber attack on the same street on 19 March 2016, which killed five people and injured 36. Turkish police later said the bomber had links to IS. In January that year, a suicide bomber in the busy Sultanahmet district killed 13. In June, attackers armed with automatic weapons and explosives belts attacked the entrance of Istanbul’s Atatürk airport, killing 45 people other than the attackers and injuring more than 230. “Without knowing who is behind this attack, the fact that this is the first terror attack in six years brings back terrible memories of the 2015-16 period when hundreds died across Turkey,” said Soner Cagaptay of the Washington Institute for Near East Policy thinktank. “I think we could see a hardening of the electorate to the right as a result,” he added, pointing towards elections due to be held in Turkey next year, if not earlier. “I think the same trend line could hold again, that whoever portrays themselves as a security candidate standing against terrorism could consolidate their base.” Cagaptay said the Turkish authorities’ efforts to throttle social media and clamp down on alleged disinformation could backfire, with the chilling effect of government restrictions visible across domestic media. “I think it’s a troublesome trend as people want to get news. Whenever you crack down on press freedom in a breaking news environment, where mainstream outlets are prevented from covering this, the trend is that people go to alternative sources. There’s always a danger of fake news spreading whenever the government tries to clamp down on news stories,” he said.