Turkish police have apprehended more suspects in connection with the bombing of a bustling pedestrian avenue in Istanbul that killed six people and wounded several dozen others, bringing the number of people in custody to 50, Turkey's justice minister said on Tuesday.

Sunday's explosion targeted Istiklal Avenue a popular thoroughfare lined with shops and restaurants and was a stark reminder of bombings in Turkish cities between 2015 and 2017 that crushed the public's sense of security.

Turkish authorities blamed the attack on the outlawed Kurdistan Workers' Party, or PKK, as well as Syrian Kurdish groups affiliated with it. The Kurdish militants groups have, however, denied involvement.

Police carried out raids in Istanbul several hours after the blast and detained 48 people, including a Syrian woman who is suspected of leaving a TNT-laden bomb at Istiklal.

Police said the woman, identified as Ahlam Albashir, had crossed into Turkey from Syria illegally and has admitted to carrying out the attack.

On Tuesday, Justice Minister Bekir Bozdag said the number of suspects in custody has increased to 50, but did not provide details. The state-run Anadolu Agency said police on Tuesday detained two brothers, identified as Ammar J. and Ahmed J.

Ammar J. was allegedly tasked with helping Albashir flee Istanbul to neighbouring Greece after the attack, while Ahmed J. allegedly drove a suspect who is still at large, to Edirne province, near the border with Bulgaria, Anadolu reported.

"Turkey continues with its fight against terrorism with determination," the independent T24 news website quoted the minister as saying. "No terrorist organisation will succeed in any kind of plot against Turkey."

Around 80 people were hospitalised following the attack, of whom at least 57 have been discharged. Six of the wounded were in intensive care and two of them were in serious condition, officials said.

The six killed in the blast were members of three families and included two girls, ages 9 and 15.

The Kurdistan Workers' Party, or PKK, has fought an armed insurgency in Turkey since 1984. The conflict has killed tens of thousands of people since then.

Ankara and Washington both consider the PKK a terrorist group, but disagree on the status of the Syrian Kurdish groups, which have been allied with the US in the fight against the Islamic State group in Syria.

Turkey has been infuriated by US support for the Kurdish militia in Syria, and on Monday, Interior Minister Suleyman Soylu said he rejects messages of condolences from Washington.

President Recep Tayyip Erdogan, however, posted on social media a chart with the flags of all countries that have offered their condolences including the American flag with a message expressing his "heartfelt gratitude" to all states and institutions that have "shared our grief".