A 48 year old woman was admitted with a history of weakness for two months. On examination, cervical lymph nodes were found enlarged and spleen was palpable 2 cm below the costal margin. Her hemoglobin was 10.5 g/dl, platelet count 2.7 X 1091 L and total leukocyte count 40 X 109/ L, which included 80% mature lymphoid cells with coarse clumped chromatin. Bone marrow revealed a nodular lymphoid infiltrate. The peripheral blood lymphoid cells were positive for CD 19, CD5, CD20 and CD23 and were negative for CD 79 B and FMC-7. The histopathological examination of the lymph node in this patient will most likely exhibit effacement of lymph node arachitecture by -