“Today is a celebration of democracy, a celebration of Istanbul,” he told a sea of Turkish flags in the city’s historical centre.
None of Turkey’s main television channels, seen as cowed by the government of President Recep Tayyip Erdogan, carried Imamoglu’s speech.
It is the second time this year that Imamoglu, of the Republican People’s Party (CHP), has been handed the keys to the city.
His first election victory in March was annulled after controversial claims of rigging by Erdogan and the ruling AKP.
But he won a decisive victory in the re-run last Sunday, increasing his margin of victory from just 13,000 in March to more than 800,000 against Erdogan’s chosen candidate, Binali Yildirim.
“The people of Istanbul taught a lesson to a handful of people who wanted to harm democracy,” Imamoglu said.
The 49-year-old former district mayor has electrified the opposition by eschewing the usual aggressive trash-talking of Turkish politics.
“He speaks a language we have been missing for years. That’s why we are here,” said Erol, a supporter in the crowd.
Erdogan – who once said that winning Istanbul was like winning all of Turkey – appears to have accepted the victory.
“We don’t have the luxury of turning a deaf ear... to the message given by our people,” he told party members on Wednesday.
The defeat is seen as especially sensitive for the Turkish leader, who grew up in one of its working class neighbourhoods and launched his political career as Istanbul mayor in the 1990s.
It comes against the backdrop of an economic slowdown and double-digit inflation which analysts say contributed to the ruling party’s losses in major cities including Ankara in the March local elections.
However, the results showed Erdogan’s party remained the most popular nationwide, with the president still credited for the economic boom of the early 2000s and for giving religious conservatives a seat at the table.
There have been fears the government may try to curtail mayoral powers in opposition-run cities.
The opposition Cumhariyet newspaper reported this week that Ankara had removed municipalities’ power to appoint the heads of city-run companies, although this has not been confirmed.
“Istanbul is a metropolis and has a vast budget. It is possible that Erdogan may try to block its funding opportunities, but... it has many opportunities to generate and develop its own funding,” Unal Cevikoz, deputy head of the CHP, told AFP.
By Fulya Ozerkan
