The leaders, who visit Chelsea in their Champions League last-16 second leg on Tuesday with a 2-1 advantage, could see their domestic lead slashed to a point if second-placed AC Milan beat visiting Chievo on Sunday.
Inter's Ghanaian substitute Sulley Muntari had only been on the pitch a minute when he committed a foul. He then handled Giuseppe Mascara's resultant free-kick to concede a penalty and was also sent off for a second yellow card.
"Inter are the strongest team in the championship but tonight we have shown that when we play with grit and determination we can beat anybody," Uruguayan forward Martinez told Sky TV.
"Until we took the lead we controlled the game then we paid for a lack of concentration," said Inter technical director Marco Branca.
"We knew Catania were enjoying a good moment of form. We weren't thinking about Chelsea, that will be another game."
He introduced winger Ricardo Quaresma, who troubled the home defence, and Milito was soon on the scoresheet after the consistently impressive Wesley Sneijder and Samuel Eto'o had combined to set up his 16th league goal of the campaign.
