The winner of Turkey's national beauty pageant was on Friday forced to hand back her crown hours after winning, over a tweet relating to the failed coup that was deemed offensive.

Itir Esen, 18, won Miss Turkey 2017 during a ceremony in Istanbul on Thursday night and was going to represent the country in the Miss World competition in China.

But organisers said that would not be possible after they discovered an "unacceptable" tweet Esen sent around the first anniversary of the July 15, 2016 coup attempt aimed at ousting President Recep Tayyip Erdogan.

In the tweet, she compared the blood shed in her menstrual cycle with that of the 249 people who lost their lives during the failed coup and are now celebrated in Turkey as martyrs.

"I am having my period this morning to celebrate the July 15 martyrs' day. I am celebrating the day by bleeding on behalf of our martyrs' blood," she wrote.

Organisers said they had seen the tweet after the beauty contest's results were announced and had to hold an hours-long meeting to verify the post.

Can Sandikcioglu, head of Miss Turkey, said in a statement that the tweet was posted by Esen.

"The Miss Turkey organisation, whose objective is to promote Turkey in the world and to contribute to its image, cannot accept such a post," he announced.

Esen's title was handed to Asli Sumen, who came second and who will now represent Turkey in the Miss World contest in China on November 18.

In a message shared on her Instagram account, Esen said her post was written "carelessly" but was not politically motivated, apologising for any misunderstanding.

"I want to say that as a 18-year-old girl, I had no political aims while sharing this post," she wrote, saying that she only shared her "innocent thoughts during a sensitive time."

"My family raised me by teaching to respect our homeland and nation. I do not have a character that could show disrespect to our martyrs," she said.

Esen is not the first Turkish beauty queen to fall foul over a social media post.

In 2015, prosecutors launched an investigation against former Miss Turkey beauty queen Merve Buyuksarac on charges of insulting Erdogan through social media posts.

Buyuksarac was given a 14-month sentence by an Istanbul court last year, but the sentence was suspended on condition that she does not reoffend within the next five years.

Opposition critics of the president have decried shrinking freedom of expression under Erdogan in Turkey, where thousands have been prosecuted for social media postings deemed to have insulted the Turkish leader.