He is also "dismayed" by reports that human rights activists have been intimidated and stripped of their citizenship, a UN statement said.
Bahrain's authorities have suspended the main Shiite opposition group pending a ruling on its dissolution.
Human rights defender Nabil Rajab, founder of the Bahrain Centre for Human Rights, has been re-arrested and a jail sentence for Al Wefaq's leader Sheikh Ali Salman on charges of inciting violence has been more than doubled to nine years.
Ban said "the current actions against the opposition may undermine the reforms undertaken" by King Hamad ibn Isa Al Khalifa and "lessen the prospect of an inclusive national dialogue in the interest of all people of the kingdom.