He said after a Legislative Council meeting that visitors delivered "inciting words" to inmates and supplied them with treats that would help them "establish superiority" in jail.
"Even if they get hold of one hair clip, or a piece of chocolate, it is regarded as a privilege, which can then recruit followers and establish influence, and spread more hatred against the government," Tang said.
The "inciting words" also flowed into prisons through letters, and Tang slammed the 612 Humanitarian Relief Fund for sending letters to inmates.
"This will severely affect the rehabilitation programs of the persons in custody, and will spread the seeds of endangering national security."
He also lashed at people for "abusing" the Ombudsman complaint mechanism, but "the Correctional Services Department will be professional and fearless as usual," Tang said.
Last Thursday, former district councillor Tiffany Yuen Ka-wai was among 18 prisoners probed by the department after it pressured the Lo Wu Correctional Institution to rescind disciplinary action taken against six other inmates who were found with prohibited articles.
Yuen was remanded after being charged with subversion for taking part in the pro-democracy camp's primary elections last July.
Former social welfare sector lawmaker Shiu Ka-chun later confirmed that Yuen was placed under solitary confinement after the unrest.
Agricultural technology, big data, and other tech sectors continued speedy growth, according to a new report on the global state of entrepreneurship.