A heavily pregnant woman has lost her baby after she was repeatedly kicked in the stomach in a "sickening" assault in the street.
The 21-year-old, who was 32 weeks pregnant, was attacked by two men wearing motorcycle helmets in Peckham, south-east London, on Monday night.
Scotland Yard said there was a "strong likelihood" the men were trying to kill the unborn child and the attack was being treated as "akin to murder".
A 20-year-old man from south London, who was arrested on suspicion of child destruction and grievous bodily harm, is being questioned by police.
Detective Chief Inspector Robert Pack, from the Homicide and Major Crime Command, said: "This was a truly abhorrent and vile attack on a heavily pregnant woman who has now, tragically, lost her baby.
"The men who attacked her must have known the impact their actions would have when they kicked her repeatedly in the stomach.
"It is sickening to think someone would deliberately target a pregnant woman in this way."
He added: "Certainly we're treating it akin to murder."
Scotland Yard said the "brutal assault" happened as the woman walked home along Talfourd Place in Peckham at around 8pm.
She was approached by two men who pushed her to the ground and continually kicked and stamped on her stomach.
The suspects - who are described as black and in their 20s - then ran off, still wearing their helmets, in the direction of Denmark Road.
A number of people heard screams and witnessed the attack outside a converted pub and rushed to the woman's aid after the men fled, before a member of the public alerted police.
Officers administered first aid at the scene before the victim was rushed to a south London hospital where she lost her baby in the early hours of the morning.
Mr Pack said the woman, who is also black, remained in a critical condition in intensive care but she had managed to provide an account of the attack to officers.
"From what we know at the moment, the working hypothesis is that it was targeted at that unborn child," he said.
"There doesn't appear to be any blows or physical force directed to her head or face. It seems to be all the violence was targeted towards the child."
He added: "There's absolutely no suggestion this is part of a pattern or spate of similar attacks. This is a one-off."
Asked whether the victim knew her attackers, Mr Pack replied: "Quite possibly."
Mr Pack told reporters that murder detectives were dealing with the case because of the "devastating impact" on the woman's life and the "sheer level of mercilessness involved".
A motorcycle has not been recovered by police and it was possible the attackers were wearing helmets as a disguise, he said.
Mr Pack added: "I have absolutely no doubt that someone out there has either heard something, or overheard something, or had someone confide in them.
"It's that sort of person that really needs to search their conscience and think 'This is not on, this is beyond the pale'."
Anyone with information is asked to contact the incident room on 020 8345 3985 or Crimestoppers anonymously on 0800 555 111.