A Saudi journalist critical of the ultra-conservative kingdom's assertive crown prince has reportedly gone missing in Istanbul, with fears he may have been abducted by the regime.

Jamal Khashoggi, who has been vocal about his criticisms of Crown Prince Mohammed bin Salman's policies, disappeared when visiting the kingdom's consulate.

He went inside the building yesterday while his fiance waited outside to get proof of divorce from his Saudi wife so he could remarry in Turkey.

His fiance, Hatice Cengiz, was left waiting for him to come out, but he never emerged from the compound, which has been sealed off with barricades.

Jamal Khashoggi, who has criticised the regime in Washington Post articles, has not been seen since entering the Saudi consulate yesterday afternoon

His fiancee Hatice Cengiz (left) and her friend wait outside Saudi Arabia's consulate to find out what happened to Mr Khashoggi

The 36-year-old said while still waiting outside today desperate for information: 'I don't know what's happening. I don't know if he's inside or if they took him somewhere else.

'We want to know his whereabouts. Where is Jamal? We want him to come out of the consulate safe and sound, the same way as he came to Turkey.

'He is a well-known writer, an internationally recognised writer not only in the United States but in Germany, Britain and the European Union.'

Turan Kislakci, of the Turkish Arab Media Centre, who has been helping her, said he believes Mr Khashoggi is still inside the building.

He said: 'She is very sad about his situation, she doesn't know what is going on inside, she is very anxious about his safety. Their marriage was supposed to be finalised this week.

'He had no specific concerns about the Saudi authorities before he went inside. We think he will be released in a couple of days.'

On his personal website a message reads: 'Jamal Khashoggi has been arrested at the Saudi consulate in Istanbul.'

Turkish authorities reportedly contacted their Saudi counterparts at around 9pm last night local time, warning security forces had surrounded the consulate.

A Saudi diplomatic vehicle leaving the Saudi Arabian consulate yesterday. Mr Khashoggi is believed to still be inside around 24 hours after he went to collect divorce papers

Saudi Crown Prince Mohammed bin Salman (left) in Kuwait last month with Kuwaiti Emir Sheikh Sabah al-Ahmad al-Jaber al-Sabah

He left his fiance outside with his phone and instructions to call if he did not emerge.

A friend of the writer said embassies throughout the Middle East routinely require phones to be left outside as a security precaution.

Mr Khashoggi, who writes opinion pieces for the Washington Post, has not been seen since entering the consulate yesterday afternoon at around 1pm, according to his fiance who accompanied him but waited outside until it closed.

The 59-year-old was a former government adviser who went into self-imposed exile in the United States last year to avoid possible arrest and has been critical of some of the policies of Saudi Crown Prince Mohammed bin Salman and Riyadh's intervention in the war in Yemen after initially supporting the military action.

Since his exile he has written columns in the Post criticising Saudi Arabia's policies towards Qatar and Canada, as well as the crackdown on the media and activists.

He said before he left the kingdom the Saudi government banned him from Twitter 'when I cautioned against an overly enthusiastic embrace of then-President-elect Donald Trump'.

Saudi consulate in Istanbul. He is believed to have been held over his criticisms of the Saudi regime

Jamal Khashoggi has been held in the Saudi consulate in Istanbul since Tuesday afternoon. Her has been critical of the government's crackdown dissent

Galip Dalay, a friend of Mr Khashoggi, is worried he could suffer torture and the hands of the Saudi authorities.

He said: 'We are very concerned as in Saudi Arabia there is no rule of law, his life is in danger and all his supposed crime is that he wrote pieces in the Washington Post.

'This is Saudi Arabia, we fear he could be tortured. He is one of the few intellectuals who speaks his mind and speaks up against the government.

'It's a very uneasy time we are going through, he has not been arrested for any specific reason, they knew he was in Istanbul and he was coming to the consulate to collect some papers, so I think they prepared for this and took their opportunity as they knew he was coming.'

Former Saudi newspaper editor, Mr Khashoggi has lived in Washington DC, for more than a year after he said the Saudi authorities had told him to stop tweeting.

In a Global Opinions piece for the Post in September last year, Mr Khashoggi wrote: 'When I speak of the fear, intimidation, arrests and public shaming of intellectuals and religious leaders who dare to speak their minds, and then I tell you that I'm from Saudi Arabia, are you surprised?'

Missing journalist Jamal Khashoggi's Turkish fiancee Hatice (left) and her friends wait in front of the Saudi Arabian consulate

Saudi journalist Jamal Khashoggi wrote last year following his self-imposed exile, 'I have left my home, my family and my job, and I am raising my voice'

He wrote in September 2017: 'I have left my home, my family and my job, and I am raising my voice.

'To do otherwise would betray those who languish in prison. I can speak when so many cannot.'

The Post's international opinions editor Eli Lopez said: 'We have been unable to reach Jamal today and are very concerned about where he may be.

'We are monitoring the situation closely, trying to gather more information. It would be unfair and outrageous if he has been detained for his work as a journalist and commentator.'

Ali Shihabi, head of the Arabia Foundation in Washington, which regularly supports Saudi policy, expressed concern on Twitter about the reports of his disappearance.

A Saudi official closes the gates to the consulate. Jamal Khashoggi has reportedly been inside the building since yesterday afternoon

He said: 'Jamal and I have not seen eye to eye on many issues but having him go missing like this is awful.'

The US State Department said it was investigating, with a spokesman adding: 'We have seen these reports and are seeking more information at this time.'

During his career as a journalist, Mr Khashoggi interviewed Al Qaeda leader Osama bin Laden several times in Afghanistan and Sudan, and served twice as editor of Al Watan newspaper.

He advised Prince Turki al-Faisal, former Saudi intelligence chief and ambassador to the United States and Britain, and has also been close to billionaire investor Prince Alwaleed bin Talal.

Under the previous Saudi king, Abdullah, Mr Khashoggi was a correspondent who travelled with the royal family.

Jamal Khashoggi outside the BBC after appearing on the World Service three days before disappearing

David Hearst, editor in chief of Middle East Eye and a friend of Mr Khashoggi, said: 'This shows the recklessness of he Saudi government.

'He does not regard himself as a dissident, he is loyal to Saudi Arabia, he is proud of Saudi Arabia, he is mild in his views, but he believes the current regime is morally bankrupt.

'If they are willing to entrap someone like him, what would they do others? It really drives a coach and horses through the myth of reform under the Crown Prince.

'Turkey will not be happy this has happened on their soil, they take any attack on their sovereignty very seriously, it means there will be a diplomatic incident. Saudi Arabia has a real problem on its hands as no one will want to back down.'

Mohamad Soltan, an Egyptian-American activist who sees Mr Khashoggi regularly in Washington, told Reuters he was in the United States on an O-visa and had applied for permanent residency status.

Saudi Arabia, which ranks 169th out of 180 on an RSF World Press Freedom Index, has promoted a modernisation campaign since the 2017 appointment of Prince Mohammed as heir to the throne.

But the ultraconservative kingdom, which won praise in June for finally lifting a ban on women driving, has drawn heavy criticism for its handling of dissent.

All public protests are banned in Saudi Arabia, as are political parties. Labour unions are illegal, the media are controlled and criticism of the royal family can lead to prison.

A car leaves from Saudi Arabia's Consulate in Istanbul last night. It is thought Mr Khashoggi is still inside the building

Other critics imprisoned by the regime, include economist Essam al-Zamil, who is a friend of Mr Khashoggi.

He was charged this week with joining a terrorist organisation, meeting with foreign diplomats and inciting protests.

Scores of businessmen were detained last November in Riyadh's Ritz-Carlton hotel in a separate campaign against corruption, unnerving some foreign investors.

Most of them were released after reaching financial settlements with the authorities.

Turkey has had strained relations with Saudi Arabia and other Gulf states since Ankara stood by Qatar in a regional row.

Turkey has also worked with Iran, Saudi Arabia's arch-rival in the Middle East, to try to reduce fighting in northern Syria and Iranian and Turkish military chiefs exchanged visits last year.