is this sentence grammatical?

[runbabyrun]

I participated and won the second-class prize in the competition.

[susiedq]

I participated in and won the second-class prize in the competition.

[Rover_KE]

Also 'I participated in the competition and won the second-class prize'.

Rover

[mrsd]

Technically, your sentence is correct. However, it is somewhat awkward. The final phrase would read better if it followed participated.