The opposition was voiced through the Coalition of Flight Attendants, which represents 90,000 flight attendants around the country.
"Given this week's deadly terrorist attacks in Boston, this is not the time to be relaxing homeland security rules that protect the lives of the public, airline passengers and airline crews," it said in a statement. "It would be irresponsible to go forward next week with these ill-conceived changes in rules that were put in place after the 9/11 terrorist attacks for good reason.
"Secretary (Janet) Napolitano, we call on you to act now to protect the American public before this new policy takes effect next week. Do not provide an open invitation for attacks against passengers and crews on our planes.
"Allowing knives on planes ... would send a message to the country that this administration is oblivious to the safety of the traveling public. You must keep knives out of the aircraft cabin. We demand an answer now."
The new policy will allow knives with blades no greater than 2.36 inches in length aboard aircraft. The policy takes effect Thursday.