Four people, Pc Keith Palmer, the man responsible for the tragedy and two members of the public, died in the incident, which has left around 40 people hurt.
Northumbria, Durham and Cleveland have all changed their profile pictures on social media to the Thin Blue Line Union Jack, used by police in tribute.
Flags at police stations have also been lowered to half mast.
Northumbria Police said: "For our friends and colleagues in London. #policefamily."
The force's police federation, which represents officers said on Facebook: "The thoughts of all at Northumbria Police Federation and the officers we represent our with Keith's family, friends and close colleagues.
Durham Constabulary said: "Our thoughts tonight are with Metropolitan Police Service colleagues, family and friends of the victims and all those involved in today's attack in Westminster.
"If you are worried about family or friends who may have been involved in today’s incident please call the Casualty Bureau on 0800 056 0944 & 0207 158 0010. Demand will be high so please be patient."
This morning, Deputy Chief Constable Jo Farrell said: “Officer and staff have been reminded to remain vigilant whilst on patrol and consider security within police premises.
"If members of the public have any concerns then we would advise them to call 999.
"There is no specific information relating to the Durham and Darlington areas that gives specific cause for concern."
Cleveland Police said: "Our thoughts tonight are with those affected by events in Westminster, including our Metropolitan Police colleagues and family."
Tributes have also been paid in the region to Pc Palmer, 48, who died in the attack.
Heather Fagan said: "He gave his life to save others.
"Thinking of his family at this sad time, he died a hero."
Lesley Wraith posted: "RIP Keith Palmer, my thoughts go to his wife and children and family members."