Their unbroken partnership of 334 was the highest-ever against India and the fourth-highest recorded at the SCG.
Clarke was just five runs from joining Mark Taylor and Sir Donald Bradman on 334 runs, and had Matt Hayden's Australian record of 380 and Brian Lara's world mark of 400 not out within reach.
But the skipper unselfishly decided to call time and put the heat back on India with just under half the Test still remaining.
In the 100th Test played at the iconic SCG, it was fitting that Clarke would break the ground record score and post the first-ever triple hundred in Sydney.
Sehwag is one of the most devastating batsmen in the game but when the 33-year-old tried to cut Australian quick Ben Hilfenhaus (1-7), he couldn't get past an outstretched David Warner at point.
India were 1-18, but the under-pressure Gambhir, who has failed in every innings so far on tour, stood tall and saw the tourists to tea without further loss.
Earlier, India's bowlers copped a pounding and would have been relieved for the massacre to finally be over.
Zaheer Khan finished with 3-122, Ishant Sharma 1-144, Umesh Yadav 0-123 and spinner Ravichandran Ashwin 0-157.