The vessel had arrived as part of its European tour dubbed "Less Plastic, More Mediterranean".
People were invited to visit the ship on Tuesday and Wednesday and sign a petition against the invasion of single-use plastic. About 100,000 people have signed the petition to date, including about 3,000 in Croatia, the press conference was told.
The head of Greenpeace Croatia, Zoran Tomic, said that an estimated 8 million tonnes of plastic ended up in the seas annually, of which about 1,455 tonnes in the Mediterranean alone.
Tomic said that discussions were under way, involving national governments, the European Commission and the European Parliament, to amend directives concerning plastics. He said that Greenpeace wanted to ensure that progressive measures were adopted to reduce the amount of plastics and stop using single-use plastics.
Tomic stressed that plastics took centuries to degrade, adding, by way of illustration, that a disposable plastic coffee spoon which is used for a minute stays in the environment for about 400 years.
Greenpeace called on Croatian Environment Minister Tomislav Coric to introduce stricter measures to reduce plastic pollution and to focus more on multiple use and then on recycling. The organisation also wants producers to be required to participate in covering the cost of reducing the use of plastics and disposing plastic waste.
Greenpeace estimates that plastics constitute 60 to 80 percent of all marine litter, the average density of plastic litter in the Mediterranean being one piece of plastic per four square metres.
The Rainbow Warrior arrived in Rijeka on July 10 on the anniversary of the bombing and sinking of the first Rainbow Warrior off the New Zealand coast in 1985. The Rainbow Warrior was then and still is commanded by Captain Peter Willcox.
Before Croatia, the Rainbow Warrior visited Spain and Italy, and after visiting the Adriatic it will sail to Greece and Bulgaria.