IPI said it joined the Indian Newspaper Society (INS) to protest the blockade of Malayala Manorama and Mathrubhumi by activists of the CPI-M's trade union wing CITU.
"For the past 13 days, the Centre of Indian Trade Unions has been physically obstructing the distribution of Malayala Manorama and other publications of the Manorama Group in and around Calicut," IPI said.
"Distribution agents from the paper have been threatened with violence by CITU workers if they proceed with delivery.
"The blockade has since grown to include Mathrubhumi Daily. Additionally, the disruption of the supply of both newspapers has spread to Kannur district.
"Any attempt to obstruct the distribution of newspapers is a direct attack on freedom of the press and on citizens' right to information.
"IPI calls on CPI-M to unconditionally halt its blockade of Mathrubhumi and Malayala Manorama, which are both members of IPI."