Context :

Aims : We present the low frequency ( 610 MHz ) radio source counts of the VVDS-VLA field and investigate the radio spectral index properties of the sub-mJy population .

Methods : We use new deep ( r.m.s . \simeq 50 \mu Jy/beam ) observations of the VVDS-VLA field obtained at 610 MHz with the GMRT and matched in resolution ( 6 arcsec ) with already available VLA data at 1.4 GHz on the same field .

Results : We find evidence of a change of the dominant population of radio sources below 0.5 mJy ( at 1.4 GHz ) : between 0.15 and 0.5 mJy the median spectral index is significantly flatter ( \alpha = -0.46 \pm 0.03 ) than that of brighter sources ( \alpha = -0.67 \pm 0.05 ) .
A relevant contribution below 0.5 mJy from a population of flat spectrum low luminosity compact AGNs and radio quiet QSOs could explain this effect .
At even fainter flux density , between 0.10 and 0.15 mJy at 1.4 GHz , the median spectral index steepens again ( \alpha = -0.61 \pm 0.04 ) suggesting that the contribution of starburst galaxies becomes important below \sim 0.2 mJy .
Finally we present a sample of 58 candidate ultra-steep sources with radio flux density from one to two orders of magnitude lower than any other sample of such objects .

Conclusions :