No side has claimed responsibility for the explosion on Monday, Aljazeera's correspondent in Islamabad, Ahmad Zaidan, said.
Quetta is the capital of Baluchistan province where autonomy-seeking nationalists are battling the government.
Zahid Afaq, a senior Quetta police official told Reuters on Monday that:"The explosion went off near a hotel.
"The numbers of casualties could be high as injured are being brought to hospitals."
The explosion occurred in the Saryab Road neighbourhood of Quetta, the capital of Pakistan's southwestern Baluchistan province, city police official Qazi Abdul Wahid told AP.
The bomb was hidden in a black plastic shopping bag at the small tea shop, he said.
Three of the wounded suffered serious injuries and are said to be in critical condition, Zaidan said, adding that the government has not mentioned any suspects yet.
Responsibility for blasts in Baluchistan are usually claimed by the Baluchistan Liberation Army, a group which demands greater political rights for the local ethnic Baluchis and a bigger share of revenues from a province rich in natural resources, especially oil and gas, yet impoverished.
The Pakistani government has a real problem in its hand, now that operations have begun to target even civilians, Zaidan said.
Government security forces in the province are already overstretched due to the ongoing anti-insurgency operations and live in difficult conditions, he said.