Clean the crabs and remove the shell. Break off the large claws and remove the small legs. Break each body into four pieces if they are big ones or two pieces if they are small. Soak the tamarind in 3 tablespoons of hot water for 10 minutes. Strain and reserve the liquid. Heat oil and fry the cumin seeds and onion until soft. Add turmeric and red chili powder and mix. Then add the tomato paste and cook for 4 minutes. Now add the green chili, garlic, salt, chopped tomato and then crab pieces. Mix well. Cover the pan and cook over low heat for 5 minutes. Then add the tamarind liquid and cook for 20 minutes or until the crab is cooked. Garnish with chopped cilantro and green onions.