The woman, who was with her young daughter, was caught without a ticket, officials said.

After an argument with security guards she was dragged off the train and held down on a bench.

The woman was then rushed to hospital for treatment and two guards have been suspended pending an inquiry.

Police said they were also investigating the incident at Hotorget metro station as a potential case of assault, as the woman was injured.

"There are many mobile phone videos that have been posted about this that suggest that the security guards were too forceful," Henrik Palmer of Stockholm's SL public transport provider told Swedish media.

 Blogger Lovette Jallow complained the incident showed how black Swedes were unfairly targeted

While it was difficult to make reasonable judgements, he acknowledged the incident had not been handled appropriately as a whole.

The video was widely shared on social media, as many complained that a black woman was being unfairly targeted. There was shock that the woman's daughter was left crying while her mother was being overpowered by the security guards.

'Violence against non-white Swedes'

Popular blogger Lovette Jallow complained that it was no surprise that Swedes of African origin were being "racially profiled and mistreated".

"All I can do is hope the baby is alright," she wrote. She said she had been in touch with the family of the woman, who was eight-months pregnant, and that she was now home from hospital.

In a statement on Friday, the transport company told the BBC that the incident was being investigated thoroughly, but its security officers were entitled to "reject or apprehend persons who are disturbing the order".

"What we do know is that the woman was caught without a valid ticket in a ticket check and was given a penalty fare. She refused this and therefore she, according to our rules, was asked to leave the subway," an SL spokesman said.

"She refused this too and when she was going to be escorted from the subway by our public security officers she started to scream and make resistance."

Swedish feminist-anti-racist organisation Men for Gender Equality said this was not the first time that security guards had used force in recent months.

"When it comes to people of colour - non-white Swedes - we have seen there's a lot of evidence that security guards use violence and sometimes when it's totally unnecessary," said its president Alan Ali.

Suspending the guards was only a first step, he said. Both the transport authority and security company had to hold training courses on racism and reproductive health.