New Delhi, Dec. 10 -- Police officers heavily patrolled mostly empty streets in major Tanzanian cities following calls for protests over the ruling party's disputed election victory that the government declared illegal, as rights groups criticized a clampdown on free speech in the East African nation.

The results of the Oct. 29 election led to three days of violent protests that left hundreds killed, more than 2,000 detained, and vehicles and buildings burned, with protesters demanding the poll be canceled. President Samia Suluhu Hassan won with more than 97% of the vote after the country's two main opposition parties were barred and the main opposition leader, Tundu Lissu, was imprisoned.

Police trucks and officers on foot could be see...