To provide some actual numbers, Toronto's downtown area employed 420,500 in 2009, up from 385,900 in 2004. This is defined as the area roughly bounded by the Lake, Bathurst St, Bloor St (with some exceptions) and the Don River.
Yonge-Eglinton employs 20,800, NYCC 34,700. There are probably about 10,000 jobs at Yonge and St. Clair.
700,000 is not a bad guess if you included all the gaps between downtown and the Centres.
Source:
http://www.toronto.ca/demographics/pdf/survey2009.pdf It should be noted that these numbers are based on a manual survey and do not account for any undercount or employment that takes place at non-visible places of employment (home-based etc).
I can't find any definitive numbers for London and would be interested to see them. Wikipedia cites an employment level of 330,000 for the City of London, but "downtown" would cover a somewhat larger area. Plus Canary Wharf I guess.