Seven were killed at the AIC church while another three died at Central Cathedral Catholic church during the Sunday morning services.
North Eastern PC James Oleserian said among those killed are two police officers adding that those injured have been rushed to the Garissa General Hospital and other health facilities in the area.
Eyewitnesses say that four masked gunmen sprayed bullets at the AIC church congregation before hurling grenades at them.
The dead included six women, two men and two police officers.
Speaking to KBC on phone, Oleserian said security agents are in pursuit of the attackers adding that road blocks have been erected on all roads into and out of the town.
Kenya’s border region has been tense since it sent troops into Somalia to pursue al-Shabab Islamic militants.
Operations against the militant group launched last October, were designed to bring an end to cross border kidnappings and other violence which it blamed on al-Shabab.
But since then, al-Shabab has claimed responsibility for a string of grenade and bomb blasts across Kenya, many deadly. No claim of responsibility has yet been made for these latest attacks.
According to North Eastern Deputy PPO, Philip Ndolo, all 10 victims died at the AIC, while three people were injured in the apparently coordinated attack at the Catholic church.
He said five AIC attackers used a combination of grenades and gunfire – telling AP news agency that grenades were thrown into the church, then gunmen opened fire as the congregation stampeded out of the church.
Witnesses told AFP news agency that bodies lay scattered in the blood-spattered churches as scores of wounded were rushed to hospital.
“It is a terrible scene, you can see bodies lying in the churches,” regional police chief Leo Nyongesa told the agency, adding the attackers had wrestled guns off police who had been posted outside churches following previous attacks.
Ndolo said no arrests had been made.
Garissa is close to the Dadaab refugee camp, where gunmen kidnapped four aid workers and killed a driver on Friday in an attack Ndolo said he suspected al-Shabab sympathizers of carrying out.