Commonest presentation of Ca rectum is:
Bleeding is the earliest and most common symptom. Often the bleeding is slight in amount.
Alteration in bowel habit is the next most frequent symptom. An early morning urgency may occur but he passes only mucus & blood (early morning bloody diarrhea). An annular carcinoma at pelvi-rectal jn. may lead to constipation & obstruction.

Other symptoms are -

Tenesmus - almost invariably present in tumors of lower half of rectum.
Pain - is a late symptom.
Weight loss.
Diarrhoea
Constipation
Bleeding P/R
Feeling of incomplete defaecation