The Yonge Street 10K will see about 7,500 runners and walkers blast down the city's principal artery — many of them raising money for local charities.
Event organizers say they'll ask all of the runners to observe a moment of silence before the start of the race — and many of the runners say they'll be wearing something to honour those killed and injured during Monday's Boston Marathon.
Christine Bain will running with her husband. Her kids and parents are coming to watch and cheer from the crowd.
"You can't let the fear stop you ... they win then," she said.
The race will mean road closures along Yonge Street - from Erskine Avenue to Richmond Street.
Various other roads will be closed to accommodate the race as the runners continue on to the CNE.
Roads are expected to be fully reopened by 11 a.m.
A complete list of road closures can be found by clicking here.