NAIROBI — Five years after more than 1,000 people were killed in election-related violence, Kenyans on Monday began casting votes in an election seen as the country’s most important — and complicated — in its 50-year history.

Police issued alerts late Sunday of impending attacks, and the violence began even before the voting. Police in coastal Mombasa reported a 2 a.m. attack by a gang of dozens; early reports indicated several officers — perhaps four or five — were killed.

The police said late Sunday that criminals were planning to dress in police uniforms and disrupt voting.

In addition, intelligence on the Somali-Kenya border said Somali militants planned to launch attacks; a secessionist group on the coast threatened — and perhaps already carried out — attacks; and the tribes of the top two presidential candidates have a long history of tense relations.

Perhaps most importantly, Uhuru Kenyatta, one of the candidates for president, faces charges at the International Criminal Court for orchestrating the 2007-08 postelection violence.

Kenyatta, a Kikuyu who is the son of Kenya’s founding president, faces Raila Odinga, a Luo whose father was the country’s first vice president.

Polls show the two in a close race, with support for each in the mid-40-percent range.