In the last decades, several studies investigating the effect of protein on satiety showed that under most conditions, protein can be more satiating than isoenergetic ingestion of carbohydrate or fat.
This effect has been observed both after a single meal and over 24h. The effect of protein on satiety may be dependent on the type of protein ingested (animal protein, whey vs casein)
The pathophysiological mechanisms underlying satiety after protein intake seem to include stimulation of gastrointestinal hormones that affect gastrointestinal motility. Additionally, some studies suggest that ghrelin concentration, a hormone involved in regulation of food intake, might change in response to protein intake.
Source (which offers a good review on past studies): Paddon-Jones D1, Westman E, Mattes RD, Wolfe RR et al. Protein, weight management, and satiety. Am J Clin Nutr. 2008 May;87(5):1558S-1561S