The Head of the Charles Regatta is underway in Boston and Cambridge.

Nearly 12,000 athletes from around the world and more than 325,000 spectators are expected to descend on the three-mile course along the Charles River for the multi-day rowing competition.

There were concerns about a toxic algae bloom in the water ahead of the event, but health officials said Thursday that the course area is no longer under an advisory. 

Here's what to know about the world-famous regatta. 

When is the Head of the Charles?

The Head of the Charles begins Friday with races featuring rowers aged 50+. Events on Friday start at 7:45 a.m. and go through about 11 a.m.

Saturday and Sunday feature full days of races from 7:45 a.m. through about 4 p.m. or later, with the championship races happening later in the day.

Click here for the complete schedule. 

Head of the Charles course map and where to watch

The starting line is at the Boston University DeWolfe Boathouse near the Hyatt Regency Hotel, and the finish line is at Herter Park off Soldiers Field Road in Brighton.

Near the start line, spectators can watch the races get underway from the BU Bridge. For a quieter viewing experience, organizers recommend nearby Magazine Beach on the Cambridge side, where singles and doubles rowers launch their boats. 

There are seven total bridges that cross the regatta course. The River Street Bridge and Western Avenue Bridge give spectators a view of the straightest section of the river known as the "Powerhouse Stretch." 

The Weeks Footbridge by Riverbend Park is one of the most popular viewing spots. That's where rowers make a 90-degree turn, and organizers say it can be difficult to navigate if multiple crews are there at once.

The Anderson Bridge at Harvard's Weld Boathouse and the Eliot Bridge by Cambridge Boat Club are the final bridges over the course. 

Head of the Charles road closures

The city of Cambridge says the following roads are closed to cars on Saturday and Sunday from 7 a.m. to 7 p.m.:

Memorial Drive (between Western Avenue and JFK Street)
DeWolfe Street (between Mt. Auburn Street and Memorial Drive)
Flagg Street (between Banks Street and Memorial Drive)
Ash Street (from Mt. Auburn Street to Memorial Drive)

Head of the Charles parking and public transportation

Parking near the course is "extremely limited," organizers say. Harvard Stadium will have parking for about 500 cars for a rate of $40 a day. The Alewife MBTA station garage, where commuters can catch the Red Line to Harvard Square, costs $9 a day.

The Harvard Square and Central stops on the Red Line are within walking distance of the course, as is BU Central on the B Branch of the Green Line. MBTA bus routes 1, 47, 64, 66, 71, 72, 73, 74, 75 and 86 also bring riders to the course area.