Cambridge police are warning residents to keep an eye on their belongings, especially near Harvard Square, as reports of thefts have risen recently. In December, 58 percent of thefts from people in Cambridge took place in Harvard Square, according to data compiled by police. Most of the thefts involved cellphones or wallets or both. The thefts generally occurred between 8 p.m. and midnight at bars or during the late morning and early afternoon at cafes, the data said. The Harvard Square numbers were part of a citywide increase. Thefts from people increased by 14 percent in 2012, compared with the previous year.