Clinton was in Afghanistan to take part in the ceremony at the heavily fortified presidential palace Thursday, where Karzai will be sworn in for the second five-year term, the US embassy said in a statement.
"Clinton arrived in Afghanistan today for a brief visit," the statement said, adding, "She will meet with President Karzai this evening and attend inaugural events tomorrow."
This is the first time Clinton is coming to Afghanistan since taking office as secretary of state.
President Zardari also arrived in Kabul Wednesday evening, where he called on Karzai in his palace in downtown Kabul, Karzai's office said in a statement.
Security was stepped up drastically ahead of the event for fears of attacks by Taliban militants. Additional heavily armed security forces were seen patrolling city boulevards while new checkpoints were erected in various parts of the city to inspect vehicles.
Hundreds of foreign dignitaries, including heads of states and foreign ministers, are expected to attend the event.
Let's light some candles and say some prayers for our Madame Secretary's safety, shall we?