The Sensex gained over 104 points in the opening trade on Tuesday amid a firming trend in overseas markets.The 30-share index of the BSE, which closed 29 points higher in the previous session, rose further by 0.58 per cent, to 18,141.51 points in the first few minutes of trade today.In a similar fashion, the broad-based National Stock Exchange Nifty index rose by 36 points to 5,432.35 points. Meanwhile, Japan's Nikkei was up 0.49 per cent and Hong Kong's Hang Sang index rose by 0.21 per cent in early trade today. The US Dow Jones Industrial Average ended 0.57 per cent higher yesterday.