Melanie Santos and Joao Perreira will be the toast of Portugal after crowning the nation's first competitive day in the Mediterranean Games with double triathlon gold at Altafulla.

Santos won the women's race as Spain's Anna Godoy took silver here.

Federica Parodi of Italy was third, two minutes behind the winner.

Santos was never troubled after a strong swim and bike ride gave her a massive advantage.

"I am really happy that the first gold medal has been won by Portugal and also that it is in the sport of triathlon," she said. 

"At the beginning of the bike ride, I broke away with just one Spanish girl and thought 'yes, that is my opportunity'. 

"I am going to take it with all I have.

"I went really fast on the first hill and I managed to break away. 

"In the first half of the race it was not too hilly but with each lap it got harder and harder.

"It was a tough race.

"’This race doesn't have Olympic points which is sad but it is a great opportunity to see how we are going and if the training is working."

Santos has set her sights on the next Olympics in Tokyo in 2020.

"Tokyo is a big step but one that I want to take," she said.

Spain had two triathletes in the top five with Cecilia Santamaria fifth.

Spectators had lined the beachfront which had been renovated during preparations for the Games.

"This event had a particularly Spanish atmosphere," said Santamaria.

"My family were here and many others who I know also came to watch."

The Games are being streamed live outside of Spain on the Olympic Channel and are available on mobile apps.

"To see this with all the world watching was a big motivation for me," said Santamaria.

"It is all part of strategy which is to get to Tokyo, but there are others like Godoy who are also fighting for a spot."

In the men's race it was a battle royal between the Iberian countries.

Perreira was locked in a struggle with Spaniards Antonio Serrat and Antonio Lopez. 

The rivals were still close together after the cycling stage but it was Perreira who eventually made the break and crossed the line first.

Serrat was second 16 seconds adrift followed by Lopez.

In the swimming pool, Italy upstaged Spain's Rio 2016 champion Mireia Belmonte and just about everyone else on the opening day.

With 475 competitors in all sports, the Italians are the largest team here in Tarragona.

World 1,500 metres bronze medallist Simona Quadarella beat Belmonte in the 800m freestyle by over five seconds to start the gold rush for Italy. 

She won in a time of 8min 21.44sec.

"It is my first medal and I hope it will be followed by many others," she said.

"It is a staging post for me, I am working and swimming very well."

From then on it was almost an Italian procession, with rare exceptions coming from Serbia's Velimir Stjepanovic in the men's 200m freestyle and Andreas Vazaios of Greece in the men's 200m medley.

Italy's Silvia Scala set a new Games record of 28.33 to win the women's 50m backstroke, and Elena De Lido's time of 57.59 for the women's 100m butterfly was also a Games record.

Spain's first medal of the Games in the 200m breaststroke was a Games record from Jessica Vall, a biomedical researcher who retained her 2013 title to lead a 1-2 for Spain. 

She won from Marina Garcia with Turkey's Viktoria Gunes in third.

"It is very important for me to win because it is a competition with our greatest rivals and we have done a lot of extra in training to prepare, I am content," she said.

Vall was quick to pay tribute to the example of Belmonte for the rest of the Spanish swimming team.

"For us she is the standard bearer, she has shown us the way," she said.

Vall’s success was followed by Catalina Corro who came home to win the 400m individual medley in a Mediterranean Games best.

It was back to the familiar strains of the Italian national anthem in the women’s 4x200m relay, their eighth gold medal, though curiously, the three flag poles remained empty during all victory ceremonies at the pool.