Context :

Aims : The properties of young stellar clusters ( YSCs ) in M33 , identified from the center out to about twice the size of the bright star-forming disk , are investigated to determine possible spatial and time variations of the star formation process in this Local Group blue galaxy .

Methods:915 MIR sources have been extracted from the Spitzer 24 \mu m image .
Upon inspection of H \alpha and GALEX images and exclusion of evolved AGB stars , a sample of 648 objects is selected as candidate YSCs and their luminosity function is examined .
The spectral energy distribution of each object , based on aperture photometry , is compared with Starburst99 models to derive age , mass and A _ { V } of individual clusters .
In the analysis we allow for different values of the upper mass cutoff of the stellar initial mass function ( IMF ) , the porosity of the ISM , and the dustiness of HII regions .
We also examine the influence of different dust models and include corrections for incompleteness of the IMF .

Results : We find discrete MIR sources as far as the extent of the warped HI disk , i.e .
16 kpc from the galaxy center .
Their surface density has a steep radial decline beyond 4.5 kpc , and flattens out beyond the optical radius at 8.5 kpc .
We are able to identify YSCs out to 12 kpc .
At large galactocentric radii , the paucity of luminous clusters and the relevance of hot dust emission become evident from the analysis of the bolometric and MIR luminosity functions .
The YSC mass and size are correlated with a log-log slope of 2.09 \pm 0.01 , similar to that measured for giant molecular clouds in M33 and the Milky Way , which represent the protocluster environment .
Most of the YSCs in our sample have A _ { V } \sim 0-1 mag and ages between 3 and 10 Myr .
In the inner regions of M33 the clusters span a wide range of mass ( 10 ^ { 2 } < ~ { } M~ { } < ~ { } 3 \times~ { } 10 ^ { 5 } { M } _ { \odot } ) and luminosity ( 10 ^ { 38 } < ~ { } L _ { bol } < ~ { } 3 \times~ { } 10 ^ { 41 } erg s ^ { -1 } ) , while at galactocentric radii larger than \sim 4 kpc we find a deficiency of massive clusters .
Beyond 7 kpc , where the H \alpha surface brightness drops significantly , the dominant YSC population has M < 10 ^ { 3 } { M } _ { \odot } and a slightly older age ( 10 Myrs ) .
This implies the occurrence of star formation events about 10 Myr ago as far as 10-12 kpc from the center of M33 .
The cluster L _ { FUV } ~ { } vs . ~ { } L _ { H \alpha } relation is non-linear for L _ { FUV } < 10 ^ { 39 } erg s ^ { -1 } , in agreement with randomly sampled models of the IMF which , furthermore , shows no appreciable variation throughout the M33 disk .

Conclusions :