According to the Pāli commentaries, the Buddha originally gave this instruction (of loving-kindness meditation) to monks who were being harassed by the tree spirits of a forest in which the monks were trying to meditate. After doing this meditation in the forest it is said that the spirits were so affected by the power of loving-kindness that they allowed the monks to stay in the forest for the duration of the rainy season.
It is often recited as part of religious services in the Theravāda tradition, but is also popular within the Mahayana tradition. Cultivating mettā through the recitation of the Mettā Sutta is sometimes believed to have apotropaic powers within the Theravāda tradition.
It has been reported that Buddhist monks chanted the Mettā Sutta as part of their demonstration in September and October 2007 against the military in Burma.
One skilled in good, wishing to attain
that state of peace should act thus;
one should be able, straight, upright,
obedient, gentle, and humble.
One should be content, easy to support,
with few duties, living lightly,
controlled in senses, discreet,
not impudent, unattached to families.
One should not do any slight wrong
for which the wise might censure one.
May all beings be happy and secure!
May all being have happy minds!
Whatever living beings there may be
without exception, weak or strong,
long, large, middling,
short, subtle, or gross,
visible or invisible,
living near or far,
born or coming to birth
may all beings have happy minds!
Let no one deceive another
nor despise anyone anywhere.
Neither in anger nor enmity
should anyone wish harm to another.
As a mother would risk her own life
to protect her only child.
Even so towards all living beings
one should cultivate a boundless heart.
One should cultivate for all the world
a heart of boundless loving-kindness,
above, below, and all around,
unobstructed, without hate or enmity.
Whether standing, walking, or sitting,
ling down or whenever awake,
one should develop the mindfulness;
this is called: Divinely Dwelling Here.