A young man has been fatally stabbed in Forest Gate in east London, police have said.
In the second incident, a woman in her 30s was stabbed to death in Sudbourne Road, Brixton, south London, shortly after 6.30pm.
Officers attended the location along with London Ambulance Service and paramedics from London's Air Ambulance.
Meanwhile a woman in her 20s is being held over the killing of Mr Nicolaie in Colindale.
"A male aged in his 20s has been arrested on suspicion of murder".
Formal identification and a post-mortem examination are due to take place.
A spokesman added: "Officers are in the process of informing next of kin".
The Metropolitan Police say this appears to be a domestic incident, rather than part of a "knife-crime epidemic".
The two deaths bring this year's total number in London so far - involving details that have been released by police - to around 59.