Nairobi, Nov 27 (IANS/AKI): People should become priests to follow Jesus, not to seek power, Pope Francis told Kenyan clergy during the first leg of his six-day African visit.

"We follow Jesus, to the end, to the cross, not for worldly gain," Pope Francis said on Thursday in unscripted remarks to priests, nuns and seminarians.

"Lukewarmness, half-heartedness are a very grave sin, which horrifies god," he warned.

Priests and nuns should put their hearts and souls into succouring humankind and "should never stop crying" over its woes, the pontiff said.

"When a priest or a nun's tears dry up, there is something amiss," he said.

Those called to holy orders have been chosen by Jesus to serve god's flock, Pope Francis said.

"You must serve the poorest, the outcasts, those on the margins of society, little children and old people," he exhorted them.

Earlier, thousands of people braved heavy rain to attend a mass celebrated by the Pope at a university campus in Nairobi.

Amid tight security, the Pope made a plea for traditional values during the mass, saying "the health of any society depends on the health of its families".

He also appealed to young Kenyans "to shape a society which is ever more just, inclusive and respectful of human dignity".