The 37-year-old goalkeeper, who won the World Cup with Spain in 2010, was taken to hospital on Wednesday where he is being treated.
On Wednesday evening he posted a picture of himself smiling in his hospital bed, alongside an update on his condition.
'All under control here, a big scare but everything is intact. Thank you very much for the messages and affection,' he wrote on Twitter.
Casillas had been working at the PortoGaia Training Centre on Wednesday morning in preparation for their league encounter with Aves this weekend.
Casillas suffered an an 'acute myocardial infarction' - colloquially known as a heart attack - at Porto training, but was treated quickly and is not in immediate danger.
Casillas had been training with his team mates in temperatures of around 20 degrees celsius on Wednesday morning when he collapsed, causing the session to be suspended.
He remains in hospital and is not expected to feature again for Porto in the remainder of their Primeira Liga season.
Porto confirmed the worrying news on Twitter, with the message: 'Always together! Strength, Iker!', along with an image of Casillas with fans at his unveiling after signing for the club.
The club also posted an official statement on their website, which crashed due to the number of people wanting to read an update on the legendary goalkeeper's status.
The statement says: 'Iker Casillas suffered an acute myocardial infarction during Wednesday morning training session held at the PortoGaia Training Centre in Olival.
'The work session was promptly interrupted to provide assistance to the Porto goalkeeper, who is currently at the Hospital CUF Porto. Casillas is well, stable and his heart problem solved.'
Casillas burst on to the scene when he made his debut for boyhood club Real Madrid as a teenager in 1999. He remained at the Bernabeu for the following 16 years.
During his club career he has won the Champions League three times - with Real in 2000, 2002 and 2014. He also won five La Liga titles and two Copa del Reys.
The Spanish giants released an official statement, which read: 'Real Madrid wants to send all out support to beloved captain Iker Casillas.
'Casillas has taught us throughout his professional career to overcome the most incredible challenges to enhance the glory of our club. He has taught us that giving up does not fit into our philosophy of life and has shown us countless times that being stronger the harder the challenge is, the way to achieve victory.
'Real Madrid want to see our eternal captain recover as soon as possible and send him all the courage of the world.'
Such is Casillas' standing in football, Real's great rivals, Barcelona and Atletico Madrid, also sent supportive messages to the goalkeeper.
His crowning achievement came in 2010 when he became the first Spanish captain to lift the World Cup, after they beat Holland 1-0 in the final in South Africa.
Playing every minute of Spain's seven games, Casillas conceded just twice, and kept clean sheets in all four of their knockout matches.
He was central to the dominant Spain side of the era, winning both Euro 2008 and Euro 2012. He is also his country's record appearance holder, with 167 caps.
Casillas moved to Porto in 2015 and helped them win the Portuguese league title last season. He remains their No 1 goalkeeper and played both legs of the Champions League quarter final defeat by Liverpool last month.
In total, Casillas has made more than 1,000 professional career appearances, and had no immediate retirement plans, with his contract at Porto set to run until 2020.
Tributes and well-wishes have been sent from around the world of football, including by one of his successors as Real goalkeeper, Thibaut Courtois.
His former Real team mate, Sergio Ramos, tweeted three praying emojis, while fellow goalkeeper Loris Karius said he was 'shocked at the news'.
Pepe Reina, who was a team mate of Casillas in the Spain 2010 World Cup squad, tweeted a picture of the two goalkeepers embracing.
He wrote: 'With that heart so big I have no doubt that everything will be OK brother. Much courage!', along with a heart emoji.
Gareth Bale, another former Real team mate of Casillas, sent his best wishes, as did hundreds of other players - including Fabrice Muamba, who had a heart attack while playing for Bolton in a 2012 FA Cup match.
Stars from outside football also wished him the best - Spanish tennis legend Rafael Nadal tweeting: 'A huge hug to Casillas right now.
-Daily Mail