Champions Celtic were held to a 1-1 draw at Dundee United in an enthralling Scottish Premier League clash at Tannadice.

Celtic led through a Paul Hartley deflected shot in the 54th minute before United's Spanish striker Francisco Sandaza flicked in a neat equaliser with 11 minutes left.

In an incident-packed second half, referee Charlie Richmond had to be replaced by fourth official Iain Brines after sustaining a suspected broken ankle bone after a collision with Celtic's Japanese midfielder Shunsuke Nakamura.

The first half was a frustrating affair with no clear cut chances but the second half burst into life with Hartley's first league goal for Celtic.

Nakamura teased the ball into the box on the right then cut it across to Hartley who swept in a low shot from the edge of area which deflected off defender Garry Kenneth and flew away from Polish keeper Lukasz Zaluska into the far corner.

United were denied a penalty by Richmond when Roy O'Donovan was sent sprawling by a clumsy late slide tackle from Gary Caldwell.

Just five minutes later Richmond hobbled off the pitch but, despite treatment on the sidelines, he was unable to return and Brines took charge.

United levelled through substitute Sandaza as he neatly flicked a near post ball by Craig Conway from four metres over the head of Celtic's Polish keeper Artur Boruc and high into the net.

United manager Craig Levein told Setanta television: "We might have missed the penalty (kick), but it was a penalty."

Celtic now have four points from two games while rivals Rangers, who are joint top of the standings with Hamilton Academical, have six from two following their 2-0 win over Hearts on Saturday thanks to goals from Kyle Lafferty and a stoppage time penalty by Kris Boyd.