An Australian activist has been arrested and detained in London just 20 minutes into a protest outside the Chinese embassy after authorities alleged he had emailed a bomb threat.

Drew Pavlou, a 23-year-old Australian activist who has been critical of China's human rights record, held a small protest outside the embassy, displaying a number of flags.

They included those of Tibet, Taiwan, and the Kökbayraq, a flag used to represent the Uyghur inhabitants of Xinjiang.

He had intended to glue the flags to the gates outside of the embassy, a similar tactic that groups like Extinction Rebellion have adopted in recent years.

Twenty minutes into his protest, however, police arrived and arrested him, with officers alleging that Pavlou had earlier emailed a bomb threat to the embassy.

Speaking to the ABC, Mr Pavlou said he believed the case was unfounded and due to misdirection. He denied all allegations against him.

While in custody, Mr Pavlou alleged that UK police told him he would be unable to contact anyone as they continued their investigation.

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"You can't speak to a lawyer. You can't ring anyone. You can't talk to anyone. You can't tell them what's happened to you," he said.

"I was obviously just really frightened. I didn't know what I could do."

He says he was roughly handcuffed in what he called a stress position by arresting officers, and was detained for hours before being told of the charges against him.

"I said, what? What do you mean? And then it suddenly made sense why I was being handcuffed so harshly and being basically treated so poorly," he said.

Mr Pavlou told the ABC that officers informed him that the Chinese embassy had made a report against him, citing an email they received with his name "all over it" which threatened a bomb attack.

Allegedly denied consular access

Sitting in a cell by himself, with no one knowing his whereabouts, Mr Pavlou says officers told him they would be searching his UK residence and his laptop.

He requested a lawyer, but says it wasn't until midnight that he was allowed to call anyone.

By that point, all of his contacts were asleep.

"I said please put me through to the Australian consular authorities," Mr Pavlou said.

"[The officers] were like, oh, sorry, the consulate's closed. They're all asleep."

Mr Pavlou says the reason officers gave for denying him outside contact was to ensure he couldn't ask anyone to destroy evidence, although he believes that police were purposefully putting him in a stressful situation.

By midnight, he said he had yet to even see the email in question. After having spent nearly 10 hours in custody, and with no outside contact, he said he requested officers conduct an interview so that he could see the email.

Mr Pavlou alleges that officers made him sign a document, waiving his right to legal representation.

He says officers agreed to conduct an interview at around 1am, but nothing happened until 2am, at which point he says he fell asleep, only to be awoken close to 4am to do the interview.

"I said, can we do it later? I've changed my mind. I'm tired. It's 4am. And then they said 'no, it has to be now'. I was just absolutely exhausted.

Mr Pavlou says officers began by asking him who he was and why he was in the UK, his reasons for going to the Chinese embassy.

They also posed questions about how he sends emails, and if he had any mental health conditions.

He tried to explain his history of activism to the officers, though says they weren't convinced. 

"I was just begging them to believe me," he said.

Emails sent to lawyers and politicians

Mr Pavlou says officers requested the PIN for his cell phone which had been seized, and that if he didn't comply, he could have faced an additional five years for obstructing justice.

He was eventually released but says UK authorities advised him against leaving the country, despite not being formally charged with anything. Officers returned his passport, he says, but not his cell phone.

The embassy was not the only recipient of emails appearing to be sent by Mr Pavlou. 

Caoilfhionn Gallagher, a prominent human rights lawyer in the UK, received an email purporting to be from Mr Pavlou, as did two of her colleagues.

Australian Senator James Patterson also received an email, as did UK human rights campaigner Luke de Pulford, from a sender pretending to be Mr Pavlou asking for help with an arrest and assault charge.

A spokesperson from the Department of Foreign Affairs and Trade told the ABC that DFAT has offered consular assistance to Mr Pavlou, and that officials from Australia's High Commission in London would raise Mr Pavlou's claim that he was denied consular access before being released with UK authorities.

Alan Crockford, spokesman for the Metropolitan Police, refused to confirm that Mr Pavlou had been arrested, but added that "strict codes of practice under the Police and Criminal Evidence Act" are upheld when people are detained.

In a subsequent email, Mr Crockford confirmed that a man had been detained at the Chinese embassy "due to his suspicious behaviour", without naming Mr Pavlou.

"He was found to be in possession of a quantity of glue and had attempted to glue his hand to the outside of the embassy building.

"The man was arrested on suspicion of trespass on diplomatic premises, communicating false information to make a bomb hoax and criminal damage," Mr Crockford said, adding that the offering of legal advice was recorded in the man's custody record.

"He has been bailed to return pending further enquiries (sic)."

The ABC has also reached out to the Chinese embassy in the UK for comment. 

Mr Pavlou has retained legal council in the UK.