His pledge - part of President Emmanuel Macron’s plan to beef up law and order - came a day after two officers were attacked on the outskirts of Paris.
“Everybody in every district has the right to lead a normal life,” Castex told an officer on Friday.
“We’re in a tough fight but we've had some successes. We are grateful for what has been done and we come with concrete proposals, not just encouragement.” he said.
Castex was joined in his whistle-stop tour of the city by interior minister Gérald Darmanin, justice minister Eric Dupond-Moretti and the education secretary Jean-Michel Blanquer.
The prime minister said 111 police officers would arrive from a national squad by the end of 2021 to complement an expanded local force.
Castex said more security cameras should be deployed as part of what he called the war against drugs and trafficking.
