President Recep Tayyip Erdogan is on a three-day visit and is facing criticism for his oppression of the Kurds and clamping down on freedom of speech with writers and artists.
Protest groups gathered outside Downing Street, including organisations supporting journalists such as Reporters Without Borders, the Cartoonists Rights Network International and the Index on Censorship.
Around 250 journalists and media workers are in jail in Turkey as well as the Christian pastor Andrew Brunson, who is being detained on suspicion on being involved in the 2016 coup against the President.
Turkey is ranked 31 on the most dangerous countries to be a Christian by charity Open Doors.
Matthew Reece is from the advocacy team at Open Doors and explained why President Erdogan's country is a difficult place, particularly for converted Christians: "If you, for instance, are from a Muslim background and convert to Christianity, you can be in danger from your family, from your community. Many of the converts in Turkey remain in secret; meet together in secret, worship together in secret because it is dangerous."
"However there are some indigenous Christian communities in Turkey too and they do face some levels of persecution from those around them - they sometimes have difficulties with their church buildings and do sometimes experience what we call a squeeze - some pressure on their every day lives."
Matthew Reece explained that it was since the 2016 coup that a "nationalist streak" has infiltrated the way the country is led.
He said it could be described as a "dictatorial, sort of paranoid leadership" and that it "has had a big impact on people's right to express themselves and peoples' right to live their lives in a way that might be seen contrary to what is Turkish and what is Islamic and obviously that has also impacted on Christians, amongst other minority groups as well."
He added that it was a great opportunity for Theresa May to challenge him on human rights, although most of their conversation will be about trade.
"We know that countries where there is higher level of freedom of religion or belief also tend to see the economy flourish...so that's an area where it would be in the Prime Minister's interests to really push that point because it could be beneficial to both Turkey and the UK in the long run."
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