More than 800 athletes and thousands of spectators are expected to flock to downtown Gloucester tomorrow, for the first Gloucester Triathlon.

The triathlon, which, apart from being a major sports event, also serves as a fund-raiser for the Gloucester Fishermen's Athletic Association, Wellspring House and the Boston-based Pine Street Inn — will kick off shortly before 9 a.m. at Pavilion Beach, and then carry especially through downtown but also through other parts of the city throughout the morning.

That should make an exciting show for spectators, but will also have a significant on traffic — including boat traffic through the Cut and the Blynman Bridge. Here's a primer for tomorrow's event.

The race

The "mini" or "sprint" triathlon consists of:

A one-third mile swim off Pavilion Beach beginning at 8:52 a.m.

A 15-mile cycling ride that will begin in St. Peter's Square and carries over Western, Essex and Magnolia avenues before returning to the city's downtown.

A 3-mile run that will loop twice around downtown Gloucester to cap the event. Elite participants are expected to finish shortly after 10 a.m.

The closures

Blynman Bridge: Bridge will be down, 9 to 11 a.m. Sunday.

St. Peter's Square parking lot: Closed from 5 a.m. today through 1 p.m. Sunday,

Beach Court: No parking from 6 to 11 a.m. Sunday.

Main Street: Closed between Center and Washington streets from 5 a.m. to noon Sunday.

Main Street: Closed between Prospect and Center streets from 8 a.m. to noon Sunday.

Downtown parking

Organizers encourage athletes and spectators to park at Gloucester High School.

Overflow parking will be allowed on Lincoln and Centennial avenues and at Stage Fort Park.

Numerous other public parking areas will be open, including municipal lots located off Rogers Street in front of the Gloucester House Restaurant and by Harbor Loop as well as a lot between Middle and Warren streets and behind City Hall off of Warren Street.

There is also street parking along Pleasant Street and Dale Avenue.

Who's in it?

The race includes a number of world-class male and female athletes, including:

Men: Janda Ricci-Munn, Gloucester's own professional triathlete and one of the triathlon's co-founders, will be one of the favorites — if he decides to compete; he was uncertain of that going into the weekend. He won this year's Cohasset Triathlon. Dean Phillips of Wenham, who co-owns the Fit Werx bike shop in Peabody, also competes nationally, and finished second in the Marlborough Triathlon — ahead of Ricci-Munn. Other contenders include 2005 Iron Man triathlon champion Tony Delogne, Australian professional triathlete and former world duathlon champion Andy Noble, and Marlborough Triathlon winner Ethan Brown, who has also competed this year in France, Austria, Texas, Washington D.C., Mexico, Oklahoma, Iowa and California this year.

Women: Alicia Kaye, a professional triathlete from Canada, and Amanda Felder, a professional triathlete and former collegiate triathlon champion, are among the favorites. Kaye finished first in the Cohasset Triathlon and second in Marlborough while Felder won in Marlborough and finished second in Cohasset.

The race also includes some 175 athletes from Cape Ann, from the elite to recreational levels. One of the runners, as part of a relay team, will be Gloucester Maor Carolyn Kirk.

Where to watch it

Organizers say the best places to view the key parts of the event will be along Stacy Boulevard and in the area of St. Peter's Square.

For more triathlon information, visit gloucestertri.com