Metta is to be friendly. If one is friendly to people without discrimination them you are developing compassion. This element does not need meditation but meditation can also be used to develop it.
What Metta meditation tries to do is to:
adjust one's perception, thoughts and views so one does not discriminate against a class of beings or particular beings - one may be classified living being as: me, friendly, indifferent, foes or any other classification and draw boundaries and identify certain attributes one favours or disfavors. Metta breaks the discrimination due to classification, attributes or groupings so one can be friendly to any being.