Armed police were called at 10:33 p.m. (2133 GMT) after a man with a knife started to attack people in London's Russell
Square, an elegant park near the site of a 2005 suicide bombing.
"But of course at this stage we should keep an open mind regarding motive and consequently terrorism as a motivation remains but one line of inquiry for us to explore," said Rowley, who is Britain's most senior anti-terrorism officer.
Police, who arrived within five minutes of being called, used a Taser electric shock gun while detaining the 19-year-old suspect. He was later arrested on suspicion of murder.
The investigation is being handled by the homicide command with support from the counter-terrorism officers, Rowley said.
The woman was treated at the scene but was pronounced dead a short time later. The other injured people - one woman and four men - were treated in hospital and three were later discharged.
A Reuters reporter at the scene said the southern part of the square, which sits at the heart of London's university area and is close to landmarks such as the British Museum, was cordoned off by police.
A forensic tent had been erected on the pavement, the scene of the attack.