BEIRUT, Lebanon — The Arab League deepened Syria’s international isolation on Sunday by imposing a battery of economic sanctions meant to sever most trade and investment from the Arab world, an unprecedented step against a member state.

The tough measures, aimed at stopping Syria’s bloody crackdown on dissidents, constitute another blow to the Syrian economy, already reeling from sanctions imposed by the European Union and the United States.

They were a psychological jab as much as an economic one, further eroding Syria’s longstanding claim to be the heart of Arabism, a claim already battered by the country’s suspension from the league two weeks ago.

For the Arab League, an organization long ridiculed as toothless, it was the second time since the Arab Spring protests began that it had acted against a member country to protect a threatened populace. But while the group invited international military intervention in Libya in March, this time its leaders made clear that sanctions were intended to avoid it.