MEXICO CITY (Reuters) -- Tens of thousands of Mexican telephone and university workers are threatening a 24-hour strike just days before July presidential elections to pressure the government to end a long-running mining dispute.

On June 28 the workers will walk off the job, joining thousands of teachers and miners, some of whom have been on strike for months in a series of labor conflicts that have seen running street battles with riot police.

The sometimes violent conflicts have caused two deaths and dozens of injuries, creating industrial instability ahead of closely-fought July 2 presidential elections.

The telephone workers union said the June 28 strikes at Mexico's main telephone company Telmex, would affect only customer service and administrative functions. Telmex is owned by the world's third richest man, Carlos Slim.

"If you want to make a phone call you won't have any problem, but if you have to make a payment or your phone is broken there will be nothing you can do about it that day," union spokesman Eduardo Torres said on Monday.

The powerful electricity workers union is expected to organize protests to support the strike, although its members will not walk off the job completely.

Mexico's mining industry has been in turmoil for most of this year with sporadic and sometimes drawn-out strikes in favor of union boss Napoleon Gomez, accused of fraud by the government and some workers.

Torres said the one-day strike had been called to pressure the government to reinstate the ousted union boss. He said the June 28 labor action would only be called off if the government backed down.