A noisy protest was staged outside Downing Street ahead of Turkish President Recep Tayyip Erdogan's meeting with Theresa May.
Critics accused the Prime Minister of cosying up to a totalitarian leader for economic reasons as she prepared to host him at No 10.
Demonstrators chanted "Turkish state, fascist state" through a public address system and waved banners opposite the gates to Downing Street branding Mr Erdogan a terrorist.
Pro-Kurdish groups raised concerns about human rights abuses by Mr Erdogan's government and urged Mrs May to distance herself from the Turkish leader, who has been accused or purging opponents, sacking thousands of officials and jailing critics and journalists.
Mr Erdogan arrived in Britain on Sunday for the three-day visit and is due to meet the Queen.
Downing Street insisted Mrs May will raise human rights issues during the talks, which are expected to be followed by a press conference.
"Our close relationship with Turkey allows us to have frank discussions," the Prime Minister's official spokesman said.
"You can expect the Prime Minister to raise human rights.
"We have always been clear that we want Turkey to uphold its international obligations including respect for freedom of expression and political freedoms.
"It is also an opportunity for the United Kingdom and Turkey to build on our close co-operation on counter-terrorism, migration, regional stability, trade and in other vital areas.
"They have a close working relationship and there are many areas of shared interest which they look forward to discussing."
Kurdish woman Meryl, 27, from London - who did not want to give her surname for fear of reprisals when she visits Turkey - was among scores of pro-Kurdish protesters outside Downing Street ahead of Mr Erdogan's arrival.
"He is literally killing humans - Kurdish children, babies, every human being," she said,
"There are no human rights for Kurds in Turkey. If you are Kurdish, you get killed, you get raped."
She said her message to Mrs May was: "Stop supporting Turkey. I'm sure she clearly sees there is a war in Turkey and civilians - not terrorists - are getting killed."