Calls are mounting for the Senate vice president to resign after he compared Italy's first black Cabinet minister to an orangutan.

Roberto Calderoli, a leader of the anti -immigrant Northern League, has insisted he was only joking when he made the "aesthetic" comment about Cecile Kyenge at a League rally last weekend.

Kyenge, a Congolese-born eye doctor, has been insulted with racist slurs from the Northern League ever since she was named integration minister in April.

Premier Enrico Letta on Monday told League leader Roberto Maroni to put an end to the slurs from within his party ranks, saying they shame the entire country.

Kyenge, for her part, has said it's not for her to demand Calderoli's resignation but that Italians should reflect on the merits of public office-holders.