The opening came amid Ahmadinejad's two-day visit to the impoverished former Soviet Central Asian republic, which also is to include a meeting with visiting Afghan President Hamid Karzai on Wednesday.
Iran has sought closer ties with the ex-Soviet republics in Central Asia since they gained independence with the 1991 Soviet collapse.
Tehran has focused mostly on transport and infrastructure projects and restoring historically close cultural ties. The five-kilometer tunnel whose opening Ahmadinejad presided over cost US$39 million, of which Iran provided US$31 million, including a US$10 million grant, the AP reports.
A motorcade of officials and journalists was briefly blocked in the tunnel by heavy amounts of water on the road surface, which is not yet finished.
On Tuesday, Ahmadinejad warned that the conflict between Lebanon and Israel could trigger "a hurricane" of broader fighting in the Middle East.
Ahmadinejad made the statement at a joint news conference following talks with Tajik President Emomali Rakhmonov. The two leaders signed a joint statement declaring "that the use of force against Palestine and Lebanon is unacceptable."
Also Tuesday, Ahmadinejad held a telephone discussion with Russian President Vladimir Putin about the Lebanon crisis and the tensions over Iran's nuclear program, the Kremlin said in a statement. It did not give details.
Russia has tried to defuse tensions over Iran's nuclear program with a proposal that all enriched uranium used by Iran come from Russia, which would allow control of whether the uranium was enriched to weapons-grade. Iran has not overtly rejected the proposal, but has not accepted it.
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