Four people have been stabbed in a spate of "cowardly and senseless" attacks in north London that police believe could be linked.
Over a period of nearly 10 hours, the woman and three men were approached from behind and knifed in the back as they walked alone in Edmonton.
The victims appeared to have been selected randomly, police said. Two are in a critical condition.
Two men have been arrested on suspicion of causing grievous bodily harm.
The first suspect was detained just before 11:00 BST in Fore Street, Edmonton, with police saying that inquiries were continuing to establish if he is the person behind the stabbings.
The second man, aged in his 40s, was arrested later on Sunday at a residential address in Edmonton. Both men remain in police custody.
The Metropolitan Police said the motive "appears to be solely to inflict harm" as none of the victims were robbed and had not engaged in conversation before being attacked.
A spokesman said the knifeman had acted alone and "mental health issues may be a factor".
He added: "We are doing everything we can to apprehend the suspect behind these cowardly and senseless attacks."
The first attack was on a 45-year-old woman who was hurt in Aberdeen Road at 19:02 GMT on Saturday. She remains in a critical condition in an east London hospital.
Four hours later, a 52-year-old man was stabbed half a mile away in Park Avenue. His injuries are not life-threatening.
Police believe the third attack happened less than a mile away in Silver Street.
The victim, a 23-year-old man, was found injured at Seven Sisters Tube station at about 04:00 BST. He is in a critical condition in hospital.
The final stabbing happened at 09:42 in Brettenham Road, less than a mile away from Silver Street.
Police said a 29-year-old man has been taken to hospital after being stabbed in the back. He has potentially life-changing injuries.
Dervish Husseyin, 60, was at his friend's house on Aberdeen Road when he saw the woman "lying face down".
"She said they had beaten her up," he said.
Mr Husseyin said the woman had "blood on her back" but did not seem to realise she had been stabbed.
"She said she only went out for a walk on her own," he added.
Natasha Cameron, 45, who lives on Aberdeen Road, said: "I'm scared now because I always walk home [from work].
"Now I'm definitely taking the bus."
“We are doing everything we can to identify & apprehend the suspect behind these cowardly & senseless attacks. A large additional police response is now working throughout the #Edmonton area to help keep the public safe. Our advice is carry on but remain vigilant.” A/DCI Smillie pic.twitter.com/uLnVnpc3MV— Enfield MPS (@MPSEnfield) March 31, 2019
Detectives said they were searching for a slim black man, who is about 6ft 3in tall and was wearing dark clothing.
Det Ch Insp Stuart Smillie said: "Police are treating the incidents as potentially linked. The four victims are all from different backgrounds and appear to have been selected at random due to them being alone and vulnerable.
"He has approached from behind without warning."
In a video message on social media, Mr Smillie added Edmonton residents should call 999 with any information that might help catch the suspect.
Extra officers are on the streets to keep the public safe, a spokesman said.