President Recep Tayyip Erdoğan said on Thursday he was declaring a day of national mourning on Friday over Israel's assassination of Hamas political chief Ismail Haniyeh to show Türkiye's support for the Palestinian cause.

Haniyeh and a bodyguard were killed Wednesday in a predawn strike on their accommodation in Tehran, Iran's Revolutionary Guard Corps said, in an attack that has stoked fears of a wider regional conflict.

Haniyeh was in the Iranian capital for the inauguration of President Masoud Pezeshkian. The killing has sent shockwaves around the world and prompted major protests in the region, including in Türkiye.

"In order to show our support for the Palestinian Cause and our solidarity with our Palestinian siblings, a day of national mourning was declared tomorrow (Friday, Aug. 2) due to Hamas Political Bureau Chairman Ismail Haniyeh's martyrdom," Erdoğan said on social media platform X.

Although the strike on Haniyeh was widely assumed to have been carried out by Israel, Prime Minister Benjamin Netanyahu's government made no claim of responsibility and said it would make no comment on the killing.

Haniyeh has been Hamas's lead negotiator in efforts for a truce and hostage release deal for Gaza, where Israel has killed more than 39,000 Palestinians, mostly women and children, since Oct. 7.

Earlier on Wednesday, Erdoğan, who has been one of Israel's most vocal critics, condemned the assassination of his close ally and "brother" Haniyeh and said the killing would not break Palestinians' will.

He denounced it as a perfidious act and vowed to continue supporting the Palestinian cause "with all our means and strength."

"May God have mercy on my brother Ismail Haniyeh, fallen in martyrdom after this odious attack," Erdoğan wrote on X earlier on Thursday, denouncing "Zionist barbarity."

Haniyeh, who spent much time in Türkiye before the conflict started, last paid a visit to Erdoğan in Istanbul in April.

Thousands of pro-Palestinian protesters marched through the streets of central Istanbul late on Wednesday to protest the assassination.

The demonstrators held posters with Haniyeh's photos and banners reading, "Martyr Haniyeh, Jerusalem is our cause and your path is our path."

Protesters were chanting "Murderer Israel, get out of Palestine," "thousands of greetings from Istanbul to the resistance in Gaza" and waving Turkish and Palestinian flags during the march in the Fatih district of Istanbul.