"I am deeply shocked by the attacks in your capital. I once again condemn acts of terrorism, regardless of the side they are coming from, in which innocent people are dying," Mesic said in the letter.
"Allow me to reiterate my deepest conviction that the only way to successfully combat terrorism is though joint international efforts and in that sense I would like to express Croatia's readiness for such cooperation," Mesic said.
"Please offer my deepest condolences and the condolences of all Croatian citizens to the families of victims of this attack," the letter said.
In a number of explosions in London on Thursday at least 33 people died and several dozen were injured, according to the latest information of the London police.