Re: AVENGERS: Grade, Reviews, Discuss, DVD & Sequel **SPOILERS**
At the same time the alien "soldiers" were pure cannon fodder, only there for the express purpose of occupying the forces of those they were fighting while the heavier machinery did the bulk of the work for their ultimate goal. So police/military could have aided but it ultimately wouldn't help and would just be doing exactly what the Chitauri were there for, to die for the progression of their goals.
The only supervillain there that might have been able to quickly kill any of the Avengers was Loki, but he ended up being owned by Hulk rather spectacularly. Part of me wishes he had been able to do more, but that scene with Hulk was just so great not to have.