We investigate the stellar population and star formation history of the Upper Scorpius OB association, the most nearby region of recent massive star formation, over the full stellar mass range from 0.1 to 20M_{sun}_. The first part of this paper describes an extension of our large spectroscopic survey (Preibisch et al., published in 2001, Cat. <J/AJ/121/1040>) for low-mass pre-main-sequence (PMS) stars in Upper Scorpius. Using the multiobject spectrograph 2dF at the Anglo-Australian Telescope, we obtained spectra of 469 stars with magnitudes R=12.5-18.0 in a 6{deg}^2^ area, on he nights of 2001 May 16-17. Among these, we find 68 new PMS stars, nearly all of them M-type stars, by their strong lithium absorption lines. The total area covered by our 2dF survey is now 9 deg2 and contains 166 new PMS stars. Combining these results with our earlier investigation (Preibisch & Zinnecker, 1999AJ....117.2381P) yields a sample of 250 PMS stars in the mass range ~0.2 to ~2M_{sun}_.