At least 67 people were killed and scores more wounded when a small group of Al-Qaeda-affiliated fighters walked into the upmarket mall, tossing grenades and executing shoppers and staff.
Kenya has been on high alert ahead of the anniversary, especially amid fears of retaliatory Shebab attacks after their reclusive leader and the alleged mastermind of the Westgate attack, Ahmed Abdi Godane, was killed at the start of this month in a US air strike.
"We are prepared in case of anything. Specialised units are on the ground and we have intensified patrols during this period of the anniversary," Kimaiyo told reporters.
Kenyan officials, community leaders, attack survivors and relatives of the victims are due to assemble early today in Nairobi's Karura forest for a memorial procession and inter-faith prayers.
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"We will be unveiling a plaque in honour of those who lost their lives," said Rajesh Shah, chairman of Karura Forest Environmental Education Trust, one of the event organisers.
Relatives of the victims will also lay wreaths of flowers at a garden in the forest where 67 tree seedlings were planted last year. "We expect the victims' families and friends to come out and be part of the healing process," Shah said.
The commemorations will end later in the day with a candlelight concert at the National Museum, the venue of a memorial exhibition that opened this week.
The mall was crowded with hundreds of shoppers and friends meeting for a meal, as well as a children's cooking competition.
Shoppers were hunted down in supermarket aisles and killed, in what the Shebab said was revenge for Kenya's sending of troops to fight the extremists in Somalia as part of an African Union force.