PORTLAND — More than 18,000 cruise ship passengers are expected to spend part of their vacation in Portland this week as the city prepares to welcome seven ships.

Nicole Clegg, the city’s spokeswoman, said the first of seven cruise ships began arriving Sunday night, and they will continue to sail into port throughout the week.

One of the largest ships, the Caribbean Princess, is due to arrive on Tuesday morning. It will be carrying more than 3,000 passengers. Mayor Nicholas Mavodones will present the captain of the ship with a key to the city along with several locally harvested lobsters.

The largest ship to arrive in Portland this year, Explorer of the Seas, will dock on Saturday at 9 a.m. at the Portland Ocean Terminal. Explorer of the Seas will have 3,114 passengers and 1,120 crew members on board.

In an economic impact study conducted last year by the University of Maine at Orono, researchers found that in 2008 more than 47,000 cruise ship passengers from 31 cruise ships pumped between $5.8 million and $8 million into the regional economy.