Harvard University\'s embattled president, Lawrence H. Summers, resigned this afternoon and will be replaced, on an interim basis, by Derek C. Bok, who was president of Harvard from 1971 to 1991. Mr. Bok was chosen to \"clean up the mess and make conditions right for the next president,\" said a senior professor with knowledge of the tumultuous events of today in Cambridge, Mass. Mr. Summers, who had been buffeted by controversy for more than a year, was expected to resign on Monday, the professor said, and this morning\'s Wall Street Journal said his resignation was imminent. In a public letter, Mr. Summers said his resignation would be effective on June 30. \"I have reluctantly concluded,\" He wrote, \"that the rifts between me and segments of the arts-and-sciences faculty make it infeasible for me to advance the agenda of renewal that I see as crucial to Harvard\'s future. I believe, therefore, that it is best for the university to have new leadership.\"