A process suitable for large scale production of 1-[(2-bromophenyl)sulfonyl]-5-methoxy-3-[(4-methyl-1-piperazinyl)methyl]-1H-indole dimesylate monohydrate of formula, which comprises: Step (i): reacting 1-methylpiperazine of formula 1 in presence of acetic acid and aqueous formaldehyde of formula 2 at a temperature in the range of 15 °C to 35 °C for a period of 1.5 hours to 2.5 hours to obtain Mannich adduct; Step (ii): reacting the Mannich adduct with 5-methoxyindole of formula 3 in presence of methanol at a temperature in the range of 15 °C to 40 °C for a period of 2.5 hours to 3.5 hours to obtain 5-methoxy-3-[(4-methyl-1-piperazinyl) methyl]-1H-indole of formula 4; Step (iii): crystallizing 5-methoxy-3-[(4-methyl-1-piperazinyl)methyl]-1H-indole of formula 4 in toluene by heating the solution to 85 °C - 95 °C for a period of 1 hour, followed by cooling the solution to 10 °C - 15 °C for the period of 3 hours; Step (iv): recrystallizing 5-methoxy-3-[(4-methyl-1-piperazinyl)methyl]-1H-indole of formula 4 in toluene by heating the solution to 95 °C - 105 °C for a period of 2 hours, followed by cooling the solution to 10 °C -15 °C for a period of 3 hours; Step (v): reacting the above obtained crystalline 5-methoxy-3-[(4-methyl-1-piperazinyl)methyl]-1H-indole of formula 4 with 2-bromobenzenesulfonyl chloride of formula 5; in presence of tetrahydrofuran and potassium hydroxide at a temperature in the range of 20 °C to 40 °C for a period of 3.5 hours to 4.5 hours to obtain 1-[(2-bromophenyl)sulfonyl]-5-methoxy-3-[(4-methyl-1-piperazinyl)methyl]-1H-indole of formula 6; Step (vi): converting 1-[(2-bromophenyl)sulfonyl]-5-methoxy-3-[(4-methyl-1-piperazinyl)methyl]-1H-indole of formula 6 in presence of ethanol and methanesulfonic acid at a temperature in the range of 15 °C to 35 °C for a period of 18 hours to 24 hours to obtain 1-[(2-bromophenyl)sulfonyl]-5-methoxy-3-[(4-methyl-1-piperazinyl)methyl]-1H-indole dimesylate of formula 7; Step (vii): converting 1-[(2-bromophenyl)sulfonyl]-5-methoxy-3-[(4-methyl-1-piperazinyl)methyl]-1H-indole dimesylate of formula 7 in presence of aqueous ethanol and carbon slurry at a temperature in the range of 75 °C to 85 °C for a period of 0.5 hour - 1.5 hours to obtain 1-[(2-bromophenyl)sulfonyl]-5-methoxy-3-[(4-methyl-1-piperazinyl)methyl]-1H-indole dimesylate monohydrate of formula. Step (i): reacting 1-methylpiperazine of formula 1 in presence of acetic acid and aqueous formaldehyde of formula 2 at a temperature in the range of 15 °C to 35 °C for a period of 1.5 hours to 2.5 hours to obtain Mannich adduct; Step (ii): reacting the Mannich adduct with 5-methoxyindole of formula 3 in presence of methanol at a temperature in the range of 15 °C to 40 °C for a period of 2.5 hours to 3.5 hours to obtain 5-methoxy-3-[(4-methyl-1-piperazinyl) methyl]-1H-indole of formula 4; Step (iii): crystallizing 5-methoxy-3-[(4-methyl-1-piperazinyl)methyl]-1H-indole of formula 4 in toluene by heating the solution to 85 °C - 95 °C for a period of 1 hour, followed by cooling the solution to 10 °C - 15 °C for the period of 3 hours; Step (iv): recrystallizing 5-methoxy-3-[(4-methyl-1-piperazinyl)methyl]-1H-indole of formula 4 in toluene by heating the solution to 95 °C - 105 °C for a period of 2 hours, followed by cooling the solution to 10 °C -15 °C for a period of 3 hours; Step (v): reacting the above obtained crystalline 5-methoxy-3-[(4-methyl-1-piperazinyl)methyl]-1H-indole of formula 4 with 2-bromobenzenesulfonyl chloride of formula 5; in presence of tetrahydrofuran and potassium hydroxide at a temperature in the range of 20 °C to 40 °C for a period of 3.5 hours to 4.5 hours to obtain 1-[(2-bromophenyl)sulfonyl]-5-methoxy-3-[(4-methyl-1-piperazinyl)methyl]-1H-indole of formula 6; Step (vi): converting 1-[(2-bromophenyl)sulfonyl]-5-methoxy-3-[(4-methyl-1-piperazinyl)methyl]-1H-indole of formula 6 in presence of ethanol and methanesulfonic acid at a temperature in the range of 15 °C to 35 °C for a period of 18 hours to 24 hours to obtain 1-[(2-bromophenyl)sulfonyl]-5-methoxy-3-[(4-methyl-1-piperazinyl)methyl]-1H-indole dimesylate of formula 7; Step (vii): converting 1-[(2-bromophenyl)sulfonyl]-5-methoxy-3-[(4-methyl-1-piperazinyl)methyl]-1H-indole dimesylate of formula 7 in presence of aqueous ethanol and carbon slurry at a temperature in the range of 75 °C to 85 °C for a period of 0.5 hour - 1.5 hours to obtain 1-[(2-bromophenyl)sulfonyl]-5-methoxy-3-[(4-methyl-1-piperazinyl)methyl]-1H-indole dimesylate monohydrate of formula.