Kayleigh Fraser, from St Andrews, said six immigration officers turned up at her door on Tuesday claiming she had violated her visa conditions.
The 34-year-old said they demanded to have her passport “otherwise they would put me in handcuffs”.
Ms Fraser said she was then told to wait for seven days for “an investigation” to be carried out before she could retrieve her documentation.
The day before, she said she received “a panicked” phone call from an employee at the company which issued her visa telling her to leave the country immediately because authorities were investigating her.
The British citizen has been vocal on social media about ongoing protests in Colombo which, since Ranil Wickremesinghe took charge on 21 July, have been handled with “intimidation, surveillance and arbitrary arrests of demonstrators, civil society activists, lawyers and journalists”, according to Human Rights Watch.
Ms Fraser said: “They accosted me in the street and were outside my house for about 40 minutes, but they couldn’t even tell me what I had done wrong.
“They kept saying I had violated the terms of my visa, but they didn’t even know what visa I was on until they finally got hold of my passport.
“I didn’t feel safe.”
Ms Fraser video-recorded her interaction with the officials after she claimed two of them tried to barge into her home to get her passport.
She can be heard telling them she feels like she is being harassed for drawing attention to the ongoing issues faced by the Sri Lankan public, such as inflation and fuel shortages.
Since posting the video on her social media pages, Ms Fraser said activist lawyer Nagananda Kodituwakku, general secretary of the Vinivida Foundation, has been in touch with her to help her case.
She said the pair are due to meet officials at the country’s immigration and emigration department on Monday hopefully to retrieve her passport.
Ms Fraser first travelled to Sri Lanka in 2019. She spent time during the coronavirus lockdowns in Germany before returning to the island last year on a medical visa.
Ranil Wickremesinghe, the president of Sri Lanka, renewed the country’s state of emergency when he took power at the end of last month, which he reportedly said was “in the interest of public security”.
A state of emergency allows troops to arrest and detain suspects, and the president to make regulations overriding existing laws to deal with any unrest.
Ms Fraser said in the past two weeks she believes between 40 and 50 people have been arrested, with about 10 being detained yesterday alone.
“When they threatened to handcuff me it was quite frightening because under emergency law they can really do anything,” she said.
Continuing to make her voice heard about the ongoing protests, Ms Fraser added: “What is happening to these people is absolutely appalling.
“There hasn’t been enough attention on the suffering that they are going through.
“The government is using tear gas on peaceful activists, it’s horrific.
“Social media has been powerful at helping keep the people here as safe as they can be by showing more about what’s going on.”
Aberdeen-based Labour councillor Deena Tissera said she is working with MSP Foysol Choudhury and the Scottish Government to make an enquiry to the Sri Lankan government to ensure Ms Fraser’s safety and that her human rights are protected.
“Kayleigh has been getting calls asking her to immediately leave the country because her life could be in danger,” Ms Tissera said.
“She is a social media activist shedding light on the protests, state of emergency, and human right violations in Sri Lanka.
“Her British passport has been forcefully seized by Sri Lankan authorities with no given reason.”
Mr Choudhury wrote to Sarah Hulton, the British High Commissioner to Sri Lanka on Thursday to ask what steps have been taken to “retrieve her passport and ensure her safety in Sri Lanka until her return to the UK”.
A Foreign Office spokesman said: “Our staff are assisting a British woman in Sri Lanka”.
The Sri Lankan immigration authorities have been contacted for comment.