A union representing the sprawling complex on the industrial eastern outskirts of Turkey's largest city said the man was holding seven people after allowing the others to escape.
A police spokesman could not confirm the figure to AFP and the US embassy in Ankara referred all queries to Turkish officials.
"Earlier today, we evacuated our Gebze facility and are working with local authorities to resolve an urgent security situation," P&G said in a statement released to AFP.
The private DHA news agency published a photo widely circulated online of a man holding a gun with what appeared to be a suicide vest strapped to his chest.
He was standing next to a drawing of the Palestinian flag and the words "the door will be opened for Gaza" painted on the wall in red.
FILE - Governor of Kocaeli districh Seddar Yavuz speaks to press after an assailant took people hostage at Procter & Gamble located plant at Gebze District in Kocaeli near Istanbul on February 1, 2024.
AFP could not immediately verify the image, which Turkish media said came from a social media feed set up by the hostage taker.
Haberturk television said police had established contact with the assailant and were attempting to negotiate the hostages' release.
AFP reporters saw anti-riot police moving closer to the plant as the crisis stretched into the night.
'I can only pray'
Relatives of the hostages expressed frustration at the lack of information from Turkish officials or the police.
"We have been waiting for six hours without any official making a statement," the father of one hostage told AFP, requesting anonymity for security reasons.
FILE - Photo taken on February 1, 2024 showsthe Procter & Gamble plant located at Gebze District in Kocaeli.
"If you do this for Palestine, go and fight there," added Cigdem Aydemir, the mother of a 26-year-old woman taken hostage at the plant.
Like other relatives, Aydemir was following what appeared to be the hostage-taker's Instagram account for updates on the situation.
"What does my daughter have to this?" said Aydemir. "I can only pray."
The war in Gaza was triggered by Hamas's October 7 attack on Israel that resulted in the deaths of around 1,140 people, mostly civilians, according to an AFP tally based on official figures.
Militants also seized about 250 hostages. Israel says 132 remain in Gaza, including at least 29 people who are believed to have been killed.
FILE - Relatives of hostages watch on their smartphones a live broadcast made by the assailant who took an undisclosed number of people hostage inside a Procter & Gamble plant at Gebze District in Kocaeli near Istanbul on February 1, 2024.
Following the deadliest attack in Israel's history, its military launched a withering offensive on Gaza. It has killed at least 27,019 people in Gaza, most of them women and children, according to the health ministry in the Hamas-run territory.
Erdogan criticism
Turkish President Recep Tayyip Erdogan has emerged as one of the Muslim world's harshest critics of Israel because of the massive civilian death toll from its campaign against Hamas militants.
He has branded Israel a "terrorist state" and compared Israeli Prime Minister Benjamin Netanyahu to Adolf Hitler.
Erdogan has also accused the United States of supporting "genocide" in Gaza.
Erdogan's comments reflect widespread anger across the predominantly Muslim but officially secular country at the United States for its traditional support for Israel.
Hundreds of protesters stormed a southeastern Turkish air based used by US and British forces on the eve of US Secretary of State Antony Blinken's visit to Ankara in November.
Turkish online campaigns are also trying to organise boycotts of US products such as Coca-Cola and the coffee chain Starbucks.