Describing the deadly blast which left hundreds killed and wounded as regretful and worrisome, Mousavi said, "We follow up this incident and its news with great sadness."
He expressed solidarity with Lebanese government and people, emphasizing Iran's readiness for helping Lebanese brothers and sisters.
Iranian Foreign Minister Mohammad Javad Zarif also expressed sympathy with Lebanese people over the deadly blast which claimed the lives of scores of people.
He said that Iran is fully prepared to render assistance in any way necessary, adding, "Stay strong, Lebanon".
A massive explosion rocked Beirut, Lebanon, with the force of an earthquake followed by a shock wave that devastated much of the city and is believed to caused hundreds of casualties.
The blast appeared to have followed a fire that had broken out in the city's port area, based on a video from the scene. The cause of the blast was immediately unknown. The force of the blast shook buildings, which were then hit again by the shock wave that blew out windows, sending shards of glass flying.
Hospitals are said to be overwhelmed and many buildings have been destroyed.
The blast has killed at least 50 people and injured more than 2,700 others, the health minister said.