City were held to a goalless draw at the Estadio do Dragao on Tuesday night, after which Porto complained that midfielder Fernandinho should have been sent off.
An official club newsletter released on Wednesday reads: "There should have been a red card given to Fernandinho, who despite being 35 is a clear example that football money counts a lot, but it doesn't buy class or notion."
The newsletter also makes reference to City's Bernardo Silva, formerly of Porto's rivals Benfica, as "an opponent of ours who is known internationally for having been convicted of racism."
Bernardo received a one-match suspension and £50,000 fine last year for a tweet comparing team-mate Benjamin Mendy to the mascot of Spanish confectionary brand, which was condemed by anti-racism campaign groups.
In response to Porto's newsletter, a City spokesman said: “This is not the first time that Porto have reacted badly in circumstances like this.
"On this occasion it is the ill judged and targeted criticism of some of our individual players and indeed our manager, which we wholly reject.
“In 2012, the last season we encountered them, it was the denial by the club of clear racist behaviour of their fans for which they were investigated and fined. In this context, this latest outburst is almost as surprising as it is disappointing.”
Porto were fined €20,000 in April 2012 after their fans were found to have racially abused City players Yaya Touré and Mario Balotelli during a Europa League tie.
Joleon Lescott, the former City defender who was playing on the night in question, posted a response to Porto’s statement on Instagram.
“The annoying part is the racism claim,” Lescott wrote. "No racism should be ignored and trying to deflect is not going to work.
“Obviously there is a case of selective memory here, I’m sure UEFA handing them a fine for constantly targeting Mario in our game in 2012 is a just a memory. #PortoDoBetter”
Tempers also flared between the two clubs during the reverse fixture at the Etihad in October, when Porto manager Sergio Conceicao complained about Guardiola's touchline behaviour and accused City of putting pressure on referees.
The newsletter refers to Porto's 3-1 defeat in Manchester as “the scandal of the England game”.
After Tuesday's goalless draw, Conceicao also claimed that Guardiola was unhappy with the result, adding: "I would also be upset if I couldn't win with the team and the budget he has."