There were injured on both sides - grammar

Source: http://news.yahoo.com/ukraine-says-30-pro-russian-insurgents-killed-071845546.html

Avakov said Monday that pro-Russia forces in Slovyansk, a city of 125,000, were deploying large-caliber weapons and mortars in the region and there were injured on both sides. Government troops were facing about 800 insurgents, he said.

Is the way it's written grammatically correct? Do we not have to precede the word with the to denote that we are talking about a group of people?

Answer

If you would mention the injured, it would mean you are referring to a groups of injured people that you have mentioned before:

Twelve people were injured. The injured were on both sides.

However, we are not talking about a specific group of injured (people) here. In that case, we use the indefinite article, which in the singular is a, and in the plural is empty (zero-article).
Compare "I have an apple" => "I have apples".

In the same way, the writer used the indefinite article correctly like this:

There was an injured [person].
There were injured [people].

As was mentioned in the comments, injuries could well have been used instead of injured, but neither is wrong.