Can you tell me if anyone knows an online casino that regularly hosts tournaments between players? I'm interested in participating in challenges, especially when there's a chance to win not only from slots, but also from participating in ratings. It would be nice if participation was available to beginners, not just "top depositors".

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I highly recommend https://pinup-india.in/pin-up-play/ — they have tournaments every week, and sometimes more often. The conditions are fair, you can participate even with minimal bets. I myself have made it into the top 50 more than once and received prizes — from free spins to real money. The ratings are updated in real time, so you can see how you are progressing. It is especially cool to participate in thematic tournaments that are held for holidays.