Call the police
(by phone) looks reasonable in theory. But they need about 1 hour to arrive, if we have an opportunity to do it and if they have a "car available"
(I experienced that in 3 European countries.) ... which does not look like a solution giving that.
(Yet, It worked for a Brazilian girl that grabbed a thug - perhaps during one hour - until police arrive).
On the other hand, if the Police are already around the corner, they do not need a car and do not have excuses. And it worked for me. In spite of being the morning hero and arrive late (or never) to my job, I just explained to 2 cops what I was assisting
(2 guys stalking and threatening a sole girl in the tube) and they moved in the right direction.
PS: Police are faster in the US or Canada? I had a good experience in Canada, but 1 is not statistically enough...